Question

A mirror forms an image which is 30 cm from the object and twice its height, where must mirror be situated?

Answer 1

Mirror is situated at a distance of 30cm from the image.(according to your question). If you want more, then it is given below...

Assuming that a real image is formed, 
magnification = -2
image distance = -30 cm
object distance = u

$m = - \frac{v}{u}\\ \\-2= -(\frac{-30}{u} ) \\ \\ -2= \frac{30}{u}\\ \\u= \frac{30}{-2}\\ \\u =\boxed{-15cm}$

Using mirror formula;

$\frac{1}{f} = \frac{1}{u}+ \frac{1}{v} = \frac{1}{-15}+ \frac{1}{-30} = \frac{-2-1}{30} \\ \\ \frac{1}{f}= \frac{-3}{30}\\ \\ f= \frac{30}{-3}=\boxed {-10cm}$

Answer 2

No sir 30 cm is distance between image and object .  Ans is 10 cm from the object.