var notasAlunos = new Dictionary<string, Dictionary<string, List<int>>> {
    { "Ana", new Dictionary<string, List<int>> {
        { "C#", new List<int> { 8, 7, 6 } },
        { "Java", new List<int> { 7, 6, 5 } },
        { "Python", new List<int> { 9, 8, 8 } }
    }},
    { "Maria", new Dictionary<string, List<int>> {
        { "C#", new List<int> { 6, 5, 4 } },
        { "Java", new List<int> { 8, 7, 6 } },
        { "Python", new List<int> { 6, 10, 5 } }
    }},
    { "Luiza", new Dictionary<string, List<int>> {
        { "C#", new List<int> { 2, 3, 10 } },
        { "Java", new List<int> { 8, 8, 8 } },
        { "Python", new List<int> { 7, 7, 7 } }
    }}
};

double Media(string nome, string disciplina)
{
  try
  {
    List<int> notasPorDisciplina = notasAlunos![nome][disciplina];
    double media = notasPorDisciplina.Average();
    return media;
  }
  catch (System.Exception)
  {
    Console.WriteLine("Nome não encontrado");
    throw;
  }
}
Console.Write("Qual o nome do aluno que deseja saber a média? ");
string nomeAluno = Console.ReadLine()!;
Console.Write("Qual a disciplina? ");
string disciplinaAluno = Console.ReadLine()!;

Console.Write(Media(nomeAluno,disciplinaAluno));