finding how long it takes given changing acceleration

lim

1. The problem statement, all variables and given/known data
At highway speeds, a particular automobile is capable of an acceleration of about 1.57 m/s2. At this rate, how long does it take to accelerate from 72.0 km/hr to 100.0 km/hr?


2. Relevant equations

I was given: vf = vi + at

3. The attempt at a solution

Simplifying the velocities I got 28 = 1.57a
and a= 17.83 s.

learningphysics

careful about units... convert km/hr to m/s or convert m/s^2 to km/hr^2...