the mirror formula shows that 1/u+1/v=1/f where u is the object distance, v the image distance and f the focal length of the mirror.
substituting for the values from the table, that is f=13cm and u= 8cm we get:
1/8+1/v=1/13
1/v=1/13-1/8
1/v= -5/104
v= -104/5
v= -20.8
therefore the correct answer is -21cm
