Given that x(a+bx)(a-bx)=25x-4x^3, what is the value of b^(-a)? a,b>0

It's a two part question. First Part Expand the equation the left hand side to get a^(2)*x-b^(2)*x^3 thus comparing co-efficients a^2=25 and b^2=4 thus a= 5 b=2 Second Part Therefore b^(-a) is equal to 2^(-5) which is equal to (1/2)^5 which equals 1/32