Magnification is the ratio of the image's height to object's height or the ratio of image distance to object distance.
That is; Magnification = height of the image/height of object= image distance/object distance
= x/4 = 21/8
x = (21 × 4)/8
= 10.5 cm
The height of the image is 10.5 cm
The mirror in this case is a concave mirror, since when the object is placed between the principal focus and the concave mirror, it forms a virtual magnified image on the other side ofthe mirror (that is why we have a negative image distance).