A particle moves in a straight line with acceleration \(a\) m s\(^{-2}\), at time \(t\) seconds, where
$$a = 10 - 6t$$
The particle's velocity, \(v\) m s\(^{-1}\), and displacement, \(r\) metres, are both initially zero.
Show that
$$r = t^2(5 - t)$$
Fully justify your answer.
[4 marks]