Hi, I am stuck on part of the question:

G(t) = -0.04t^3 + 0.9t^2 + 30 predicts your grade out of 100% on the next calculus test when you study t hours during the week before the test.

How many hours of study required to achieve your maximum grade?

Can someone please help me with this?

I am able to differentiate but am unsure how to solvet. It doesn't fit into quadratic?