Acar slams on its brakes, coming to a complete stop in 4.0 s. the car was traveling south at 60.0 mph. calculate the acceleration. mi/h/s

We need to apply the following expression

Vf = Vo + a.t

Where a is the acceleration
t is the time it takes to achieve Speed Vf
And Vo the initial speed 

In this problem Vo = 60 miles/hour
Vf =  0 (comes to a stop)
t = 4 seconds 

a = -60 [miles/hours] / 4 [s] = -15 miles/hour/second    (negative as it goes in                                                                                                  the opposite direction)

Look it up you feel me