A projectile is launched from ground level with an initial velocity of v 0 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. Find the​ time(s) that the projectile will​ (a) reach a height of 80 ft and​ (b) return to the ground when v 0 = 96 feet per second.