A car brakes to avoid a truck stopped in the road ahead. The car brakes with a constant deceleration of$\displaystyle 16 \frac{ft}{sec^2}$, producing skid marks that measure 200 feet before it is able to stop. How fast was the car moving (in ft/sec) when the brakes were first applied?

I gathered that a=-16 and 200 will be some part of s(t). So I need a t value to plug into v(t), but how should I do it? answer is 80 ft/sec.