$11\mathrm{.}120$ Shore$-$based radar indicates that a ferry leaves its slip with a velocity $,$ while instruments aboard the ferry indicate a speed of $18\mathrm{.}4k\frac{m}{h}$ and a heading of $30$ west of south relative to the river. Determine the velocity of the river.

Fig. P$11\mathrm{.}120$

Please do $11\mathrm{.}120$