so, we have the denominators of 4 and 8, thus our LCD will just be 8, but let's firstly convert the mixed fractions to improper fractions and find their difference.
[tex]\bf \stackrel{mixed}{2\frac{3}{4}}\implies \cfrac{2\cdot 4+3}{4}\implies \stackrel{improper}{\cfrac{11}{4}}~\hfill \stackrel{mixed}{1\frac{7}{8}}\implies \cfrac{1\cdot 8+7}{8}\implies \stackrel{improper}{\cfrac{15}{8}} \\\\[-0.35em] ~\dotfill\\\\ \cfrac{11}{4}-\cfrac{15}{8}\implies \stackrel{\textit{using LCD of 8}}{\cfrac{(2)11~~-~~(1)15}{8}}\implies \cfrac{22-15}{8}\implies \cfrac{7}{8}[/tex]
