Given, $u=-1.5f$,  now use the mirror formula for the concave mirror, in terms of distance between object and pole, the distance between image and pole and focal length, 

$\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$$\Rightarrow \frac{1}{-1.5\mathrm{f}}+\frac{1}{v}=\frac{1}{-f}$

$\Rightarrow \frac{1}{v}=-\frac{1}{f}+\frac{1}{1.5f}$$\Rightarrow \frac{1}{v}=\frac{-1.5+1}{1.5f}=\frac{-0.5}{1.5f}$

$\Rightarrow v=-3f$.