Where is the function increasing where is the function

Question

Please explain and express the answer exactly, for example √3 rather than in decimal form, thanks:

s(t) = -8t⁹ + 9t⁸ - 7

(a) Where is the function increasing?

(b) Where is the function decreasing?

(c) Where does the function have local maxima?

(d) Where does the function have local minima?