College algebra

Question

A ball is thrown in the air from the top of a building. Its height, in meters above ground, as a function of time, in seconds, is given by ℎ(𝑡)=−4.9𝑡2+21𝑡+8. How long does it take to reach maximum height?

Answer 1

h'(t) = -9.8t +21 = 0

t = 21/9.8 = about 2.143 seconds to reach maximum height

just take the derivative of h(t) and set it equal to zero, solve for t