veolocity calculation

Question

A stone is dropped from the edge of a roof, and hits the ground with a velocity of -180 feet per second. How high (in feet) is the roof?Note: the acceleration of an object due to earth's gravity is 

Answer 1

y(t) = gt²/2 +v₀t +y₀

v₀ = 0

velocity, v = gt

g = -32 ft/s²

-32t = -180

t = 180/32

(-32/2)t²+y₀ = 0

y₀ = 16(180/32)² = 506.25 ft