F(x)=-x^2+7x-9 for f(2x)

I keep getting -4x^3+14x-9 for this but WebAssign won't accept that answer. I'd really appreciate some help.

f(x)=-x^2+7x-9

f(2x) = -(2x)^2 + 7(2x) - 9
f(2x) = -4x^2 + 14x - 9