$\text{Image is virtual so v = +ve and u = -ve}$

$\text{Magnification } M = -\frac{v}{u} = 3$

$\Rightarrow v = -3u , \text{ if u = -x then v = 3x}$

$  f = -18 cm $

$ \text{Mirror Formula } \frac{1}{v} + \frac{1}{u} = \frac{1}{f}$

$\Rightarrow \frac{1}{3x} + \frac{1}{-x} = \frac{1}{-18}$

$\Rightarrow x = 12cm , u = -12cm$