A projectile is launched from ground level with an initial velocity of v 0

v0 feet per second. Neglecting air resistance, its height in feet t seconds after launch is given by s equals negative 16 t squared plus v 0 t

s=16t2+v0t. Find the time(s) that the projectile will (a) reach a height of 400 ft and (b) return to the ground when v 0 is 128 feet per second.