let's firstly convert the mixed fractions to improper fractions and then get their difference.
[tex]\stackrel{mixed}{8\frac{7}{8}}\implies \cfrac{8\cdot 8+7}{8}\implies \stackrel{improper}{\cfrac{71}{8}} ~\hfill \stackrel{mixed}{6\frac{3}{4}}\implies \cfrac{6\cdot 4+3}{4}\implies \stackrel{improper}{\cfrac{27}{4}} \\\\[-0.35em] ~\dotfill\\\\ \cfrac{71}{8}-\cfrac{27}{4}\implies \cfrac{1(71)~~ -~~2(27)}{\underset{\textit{using this LCD}}{8}}\implies \cfrac{71-54}{8}\implies \cfrac{17}{8}\implies 2\frac{1}{8}[/tex]
