differentiate: $\displaystyle y=cos(a^3+x^3)$

my answer: $\displaystyle y'=-sin(a^3+x^3)(3a^2+3x^2)$

correct answer: $\displaystyle y'=-3x^2sin(a^3+x^3)$

I tried this multiple times and get the same answer (my answer). Either I am doing it incorrectly or the back of the book is incorrect.