The ball is thrown upward with the speed of 48 feet per second from the height of 64 ft. Formula y = -16x2 + 48x + 64 models ball's height above ground, y, in feet, x seconds after it was thrown. At what time ball stikes ground?
i. 3 seconds
ii. -1 seconds
iii. 1.5 seconds
iv. 4 seconds