let's firstly convert the mixed fractions to improper fractions, and then subtract.
[tex]\stackrel{mixed}{8\frac{1}{9}}\implies \cfrac{8\cdot 9+1}{9}\implies \stackrel{improper}{\cfrac{73}{9}}~\hfill \stackrel{mixed}{6\frac{7}{8}}\implies \cfrac{6\cdot 8+7}{8}\implies \stackrel{improper}{\cfrac{55}{8}}\\\\[-0.35em]~\dotfill\\\\\cfrac{73}{9}-\cfrac{55}{8}\implies \stackrel{\textit{using}\stackrel{LCD}{72}}{\cfrac{(8\cdot 73) - (9\cdot 55)}{72}}\implies \cfrac{584-495}{72}\implies \boxed{\cfrac{89}{72}\implies 1\frac{17}{72}}[/tex]
