I have a homework problem that I need help on:

A stone is dropped from the edge of a roof, and hits the ground with a velocity of -175 ft/s. How high (in feet) is the roof?

I know that the acceleration would be -32ft/s due to gravity, so v(t)=-32t+c.

However, since I don't know how long it takes for the stone to hit the ground, I don't know what to use for time, or my constant c. Can you help me to work through this problem? Thanks!