Coal is the answer to the question
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)
Make sure the triple beam balance is at 0 before you begin.