5 A random variable \(X\) has probability density function given by
$$f ( x ) = \begin{cases} \frac { k } { x - 1 } & 3 \leqslant x \leqslant 5 0 & \text { otherwise } \end{cases}$$
where \(k\) is a constant.
Show that \(k = \frac { 1 } { \ln 2 }\).
Find \(a\) such that \(\mathrm { P } ( X < a ) = 0.75\).