Question 70
The ball strikes the ground when s(t) is equal to zero, since hitting the ground is equivalent to saying that the ball is at zero height.

So, we have a problem: find t such that s(t) = 0. 

{{{-16t^2+140t+3 = 0}}}

Solve it with my <A HREF=/algebra/homework/quadratic/>quadratic solver</A>.