for a), first find t by setting f(t) = 0, then find f'(t) which is the same as v(t); in f'(t) you plug the t you found first.
for b) set v(t) = 40 and solve for t
A ball thrown straight up into the air from a bridge,and its height above the ground t seconds after it is thrown is
f(t) = -16t^2 + 108t + 48 feet.
a.Determine the velocity of the ball at the moment it hits the ground.
b.Determine all points in time after the ball is tossed that it has speed of exactly 40 feet per second(speed is the magnitude of velocity).