A proton in an accelerator detector collides head-on with a stationary alpha particle, as shown in diagrams below. After the collision, the alpha particle travels at a speed of \(4.0 \times 10^6 \text{ m s}^{-1}\). The proton rebounds at \(6.0 \times 10^6 \text{ m s}^{-1}\).
 

Find the speed of the proton before the collision, modelling the mass of the alpha particle, \(4m\), to be equal to four times the mass of the proton, \(m\). Show your working. Ignore relativistic effects.  (3 marks)