The initial velocity is -150m/s. Remember that: V(t) = Vi + at, where v(t) is velocity at time t, Vi is initial velocity, and a is acceleration. We know that the vehicle stopped after 30 seconds, so V(30) = 0. Let's plug the useful info into our function and solve for a: 0 = -150 + a*30 Add 150 to both sides 150 = 30a Divide both sides by 30 a = 5 m/s² The average acceleration is 5 meters per second squared.