How would i solve (3x-2y)^3

I keep getting the wrong answer

what do you mean by "solve"?

As it stands, there's not much to be done except expand the polynomial.

(a+b)^3 = a^3 + 3a^2b + 3ab^2 + b^3

so, you have

(3x)^3 + 3(3x)^2(-2y) + 3(3x)(-2y)^2 + (-2y)^3
= 27x^3 - 54x^2y + 36xy^2 - 8y^3