=[3-(1+1/8)+(1-1/9)]x21/13-(13/7-13/8)/13
=[(3-1/8-1/9)x21-13/7+13/8]/13
=(63-21/8-3/7-13/7+13/8)/13
=(63-1-16/7)/13
=(60-2/7)/13
=418/91