4 A continuous random variable \(X\) has probability density function
$$\mathrm { f } ( x ) = \left\{ \begin{array} { c l }
k x & 0 \leqslant x \leqslant a
0 & \text { otherwise }
\end{array} \right.$$
where \(k\) and \(a\) are constants.
- State what the letter \(x\) represents.
- Find \(k\) in terms of \(a\).
- Find \(\operatorname { Var } ( X )\) in terms of \(a\).