An object of 7cm height is placed at a distance of 12 cm from a convex lens of focal length 8cm.

Here, $h_1=7 cm$, $u=-12 cm, f=8 cm$, $h_2=?, v=?$
The formula for calculating focal length is given as,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
or, $\frac{1}{v}=\frac{1}{f}+\frac{1}{u}$
or, $\frac{1}{v}=\frac{1}{8}-\frac{1}{12}=\frac{3-2}{24}=\frac{1}{24}$
$v=24 cm$
So the position of the image is 24 cm and as v is positive, the image is real.
Now, magnification, $m=\frac{h_1}{h_2}=\frac{v}{u}$
or,  $\frac{h_2}{7}=\frac{24}{-12}$
or,  $h_2=-14 cm$
$m=\frac{h_2}{h_1}=\frac{-14}{7}=-2$