Write a function
C[p_, x_, n_] that returns the coefficient of $x^n$ in the polynomial equation.
C[p_, x_, n_] := ...
If we call
C[7 x^2 - 3 x^3, x, 2], the output should be 7. I don't want to use the
Coefficient function in Mathematica, I just want to understand how it is done.