3. A racing car moves with constant acceleration along a straight horizontal road. It passes the point \(O\) with speed \(12 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). It passes the point \(A 4 \mathrm {~s}\) later with speed \(60 \mathrm {~m} \mathrm {~s} ^ { - 1 }\).

1. Show that the acceleration of the car is \(12 \mathrm {~m} \mathrm {~s} ^ { - 2 }\).
2. Find the distance \(O A\). The point \(B\) is the mid-point of \(O A\).
3. Find, to 3 significant figures, the speed of the car when it passes \(B\).