As you guys might know I don't want an answer, I would just like some guidance.

An object is projected upward from the ground with an initial velocity of 128 feet per second. The height s at the end of t seconds is s = $\displaystyle 128t - 16t^2$.

When does it reach its maximum height and what is it?

The object reaches maximum height when the velocity is 0.

$\displaystyle v = D_x(128t-16t^2)$ = $\displaystyle v = 128-32t$

t = 4

s = 128(4)-32(4) = 256 ft

When does it hit the ground and with what velocity?

The object will hit the ground when the height is 0

Therefore $\displaystyle 128t -16t^2 = 0$

I got stuck here