If an object is propelled upward from a height of s feet at an initial velocity of v feet per second, then its height h after t seconds is given by the equation h=−16 t squared +vt+s, where h is in feet. if the object is propelled from a height of 12 feet with an initial velocity of 64 feet per second, its height h is given by the equation h=-16t squared+64t+12. after how many seconds is the height 72 feet?