For an erect image, magnification is positive. Thus, m = 3 for an erect image three times the size of the object.
m = 3
For a concave mirror,
m = \(\frac{-v}{u}\,=\,3\Rightarrow\)v = -3u
Using Mirror formula, \(\frac{1}{v}\,+\frac{1}{u}\,=\,\frac{1}{f}\)
Thus, the image is formed behind the mirror at a distance of 36 cm.