So I have the following:

$$
y = cos(a^3 + x^3)
$$

This is what I got.

$$
y' = \cos(a^3 + x^3) \ ( -sin(a^3 + x^3) ) \ ( 3a^2 + 3x^2 )
$$

I'm not sure what to do after this? Would this be the final answer?