What is the height of the image? round the answer to the nearest whole number. cm what type of mirror most likely formed this image?

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).