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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}
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