That "ad^3" bothers me; I'm not sure what you mean here.
I'm going to assume that you have (1/4*a*d^3)^2.
If that's the case, then you have (1/4)^2 * a^2 * d^6 = (1/16)*a^2*d^6.
If this is not what you were hoping for, double check to ensure that you have copied down this problem exactly as it was presented.