Algebra . Cannon ball with height and speed to hit the ground

Question

A cannon ball is launched upward with a velocity of 16 feet per second from the top of a 32 foot tall building. Its height after t seconds is given by the equation S=−16t2+16t+32. When does the cannon ball hit the ground?

Answer 1

s = 0 = -16t^2 +16t +32

divide by -16

t^2 -t -2 = 0

factor

(t-2)(t+1) = 0

set each factor = 0 and solve for t

t = 2 or -1

in 2 seconds it hits the ground. ignore the negative time