The problem is, "A stone is thrown upward with an initial velocity of 128 ft/s from the roof of a building 320 ft high. Express its height above the ground as a function of time. Find the following: a) The maximum height that the stone reaches b) The time it takes for the stone to reach the ground c) The velocity and speed of the stone when it hits the ground."

I am currently having trouble with problem b. My position equation I found to be$\displaystyle s(t) = -16t^2 + 128t + 320$.

From there, I set the position equal to zero, and then had to use the quadratic equation. The answer to b is 10 s, but I got 5 s and, I believe, -1.15 s. What did I do wrong?