6 A motorbike and its rider, together denoted by \(M\), have a combined mass of 360 kg . The resistive force experienced by \(M\) when it is in motion is modelled as being proportional to the speed it is moving at. All motion of \(M\) is on a straight horizontal road. It is found that with the engine of the motorbike working at a rate of 12 kW , the maximum constant speed that \(M\) can move at is \(10 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). Determine the speed of \(M\) such that with the engine working at a rate of 12 kW the acceleration of \(M\) is \(1.5 \mathrm {~m} \mathrm {~s} ^ { - 2 }\).