magnification m = - v/u = h'/h = +3
v = -3 u actually u is negative. Hence v is positive.
R = -36 cm => f = R/2 = -18 cm , negative for a concave mirror.
1/v + 1/u = 1/f, mirror equation
1/(-3u) + 1/u = 1/(-18)
2/(3u) = -1/18
u = - 12 cm v = +36 cm
the object is placed 12 cm from pole of concave mirror , less than the focal length, to have an ERECT magnified image 3 times the object.