In below Exercises, use the position equation

where s represents the height of an object (in feet), v0 represents the initial velocity of the object (in feet per second), s0 represents the initial height of the object (in feet), and t represents the time (in seconds).
A projectile is fired straight upward from ground level (s0 = 0) with an initial velocity of 128 feet per second.
(a) At what instant will it be back at ground level?
(b) When will the height be less than 128 feet?