Final is tomorrow brahs....I NEED an A.

I just Don't understand the steps to solving these, please show step by step what you do.

For this last one I get 5sin^4 (x^3 +3)+sin^5 (3x^2)

But he says the right answer is: 5sin^4 (x^3)cos(x^3 +3)(3x^2)

Where do you get that cos?