The brakes on your car can slow you at a rate of 5.2 m/s2. (a) If you are going 137 km/h and suddenly see a state trooper, what is the minimum time in which you can get your car under the 90 km/h speed limit? (The answer reveals the futility of braking to keep your high speed from being detected with a radar or laser gun.) (b) Graph x versus t and v versus t for such a slowing.

The brakes on your car can slow you at a rate of 5.2 m/s2. (a)
If you are going 137 km/h and suddenly see a state trooper, what is
the minimum time in which you can get your car under the 90
km/h speed limit? (The answer reveals the futility of braking to
keep your high speed from being detected with a radar or laser
gun.)

We choose the positive direction to be that of the initial velocity of the car
(implying that a < 0 since it is slowing down). We assume the acceleration is constant
and use Table 2-1.
(a) Substituting v0 = 137 km/h = 38.1 m/s,
 v = 90 km/h = 25 m/s,
to covert km/h to meter/second multiply the 137 & 90 by 0.27778 
and
a = –5.2 m/s2 (from the question)
into v = v0 + at, we obtain
t = (25-38.1)/-5.2= 2.5s