Uniform Accelerated Motion

Spotting a police car, you hit the breaks on your new porsche to reduce your speed from 90 mi/hr to 60 mi/hr at a constant rate over a distance of 200ft.

a)Find the accerleration in ft/s^2

b)How long does it take to reduce your speed to 55 mi/hr?

c)At the acceleration obtained in part (a), how long would it take for you to bring your porsche to a complete stop from 90 mi/hr?

I know this has something to do with these formulas

s(t)=So+Vot+1/2gt^2

v(t)=Vo+at

but I am not sure how to apply them and where to start.