Mirror formula
1/f = (1/p) + (1/q) [ f= focal length , p= object distance , q = image distance ]
as given convex mirror so
distance of object =p= 6cm
focal length = -8cm
to find q=?
1/q =( 1/f) - (1/ p)
1/q = (1/ 8) -(1/6)
1/q =-1/24
q=-24 ( negative sign shows image is virtual and behind the mirror)