a) The image formed in front of the concave mirror is real, so m is negative,

m = -3, u = -10 cm

As m = -v/u or -3 = -v/-10

v = -30 cm

By mirror formula,

1/f = 1/v + 1/u = 1/-30 + 1/-10 = -4/30

or f = -30/4 = -7.5 cm

Radius of curvature, R = 2f = 2 x (-7.5) = 15cm

a) The image formed in front of the concave mirror is real, so m is negative,

m = -3, u = -10 cm

As m = -v/u or -3 = -v/-10

v = -30 cm

By mirror formula,

1/f = 1/v + 1/u = 1/-30 + 1/-10 = -4/30

or f = -30/4 = -7.5 cm

Radius of curvature, R = 2f = 2 x (-7.5) = 15cm

the above answer is wrong pls check it

a) The image formed in front of the concave mirror is real, so m is negative, m = -3, u = -10 cm As m = -v/u or-3 =- v/-10v = -30 cm  By mirror formula,Thus, focal length of concave mirror is 7.5 cm.  Thus, Radius of curvature, R = 2f = 2 x (-7.5) = - 15cm

The calculations seems to be perfect. And it follows all the conditions as per the given data. If you still have any doubt regarding it you can post the complete question which will give clear idea about what information is actually given and what needs to be calculated.

As the object is placed at 10 cm it means it is between centre of curvature and focus and hence the image must be formed beyond centre of curvature i.e. 30 cm according to given data.