I need help on my homework. Please. Thank- You!?

A penny is thrown downward from the top of a 320 foot tall building at a speed of 16 feet per second. Using the formula d=vt+16t^2 (it is 16t squared), where v is the initial velocity, d is the distance in feet, and t is the time in seconds, how long will it take for the penny to hit the ground?