Since the image is real, it must be inverted and magnification ratio should be negative.

 

u =-x

M = -n = $\frac{-v}{-x}$

$\Rightarrow v = -n.x $

$\frac{1}{-f} = \frac{1}{v} + \frac{1}{u} = \frac{1}{-nx} + \frac{1}{-x}$

$ x = \frac{(n+1)f}{n}$