Given R=30cm
So, f=2R=+15cm
For convex mirror, virtual image is formed for real object. Thus, magnification (m)=21
The magnification can be written as
m=−uv...(1)
Equation (1) implies that
21=−uv⇒v=−2u
The relation between the object distance, image distance and the focal length of the mirror is given by
f1=v1+u1...(2)
Equation (2) implies that
151=−2u1+u1=−u1⇒u=−15cm