A ball is thrown at a velocity of 16ft/sec off the top of a roof 320 feet above the ground.

a) Use the formula s=-16t^2 + v0t +s0 to write the equations which describes the height of the ball above the ground, s, in t seconds after the fall begins.

s=-16t^2 + 16t +320

b) How long will it take the ball to hit the ground?

How do I figure out the answer for part b?