How to intersect two select with int and string type in LINQ Method syntax?

I am new to programming I hope you can help me to write better code.

Description

I want to write a query that prints Average scores of Californians who are active and have the letter R or D in their name.

Example Input

StudentName = {"Ben Wild", Score = 65, StudentCity = "CA", StudentActvity= true}

StudentName = {"Sara Ride", Score = 81, StudentCity = "CA", StudentActvity = true}

My problem

I need to select their name and their score at the end but I can't select two Lambda expressions and && them together.
I got this error : 'string' does not contain a definition for 'Score' and no accessible extension method 'Score' accepting a first argument of type 'string' could be

I tried so many ways that I will mention my solutions below :

Program.cs code

using System;
using System.Collections.Generic;
using System.Linq;

namespace _64_LINQ_Practice
{
    class Program
    {
        static void Main(string[] args)
        {
            //Access to the class method
            var studentList = StudentDatabase.GetStudentsFromDb();
            var studentsName6 = studentList.Where(x => (x.StudentActvity == true) && (x.StudentCity == "CA")).Select(x => x.StudentName);
            List<string> xstudent = new List<string>(studentList.Select(x => x.StudentName));
            IEnumerable<string> final = xstudent.Where(
                                x => x.Contains("R") || x.Contains("r") || x.Contains("d") || x.Contains("D"));

            foreach (string studentFinal in final)
            {
                Console.WriteLine("The students that are active, live in CA and names with r/R or D/d letter :" + studentFinal);
            }
        }
    }
}

and second way:

        var studentsName6 = studentList.Where(x => (x.StudentActvity == true) && (x.StudentCity == "CA")).Average(x => x.Score);
        List<string> xstudent = new List<string>(studentList.Select(x => x.StudentName));
        List<int> intstudent = new List<int>(studentList.Select(x => x.Score));
        IEnumerable<string> final = xstudent.Where(
                            x => x.Contains("R") || x.Contains("r") || x.Contains("d") || x.Contains("D"));
        IEnumerable<int> final2 = intstudent;
        var convFinal = final.AsQueryable();
        var convFinal2 = final2.AsQueryable();
        var final3 = studentsName6.AsQueryable().Concat(convFinal.AsQueryable()).Concat(convFinal2);
        var final4 = final3.average(x => x.Score);
        Console.WriteLine(final4);

There's no attempt to calculate averages in the code so it's unclear what the problem is. Going by

> prints the average score of students, with these conditions: Average score of students that live in CA and their activity is true and there are R/r or D/d letters contain in their name.

It seems a single Where is needed to filter the students, then Avg to calculate the average. Since there's no need to calculate averages for multiple groups, GroupBy isn't needed.

var letters=new[]{"r","d"};
var average=studentList.Where(st=>
        st.StudentActvity 
        && st.StudentCity == "CA"
        && letters.Any(l=>st.StudentName.Contains(l,StringComparison.OrdinalIgnoreCase))
     )
    .Average(st=>st.Score);

String.Contains can accept a parameter that specifies case-insensitive comparison, so there's no need to specify both small and capital letters.

letters.Any(l=>st.StudentName.Contains(l...)) checks if any letter is contained inside StudentName

This query will work with an in-memory database and probably an actual database. It will be very slow though. name.Contains("r") is translated to the SQL condition name LIKE '%r%'. There's no way to use an index to quickly find which names have r in their middle, so the database will have to search the entire table