A projectile is launched from ground level with an initial velocity of v0 feet per second. Neglecting air resistance, its height in feet t seconds after launched is given by s= -16t^2 v0t. Find the...

A projectile is launched from ground level with an initial velocity of v0 feet per second. Neglecting air resistance, its height in feet t seconds after launched is given by s= -16t^2 v0t. Find the time(s) the projectile will (a) reach a height of 64 ft (b) return to the ground when v0=80 feet per second.