let's firstly convert the mixed fractions to improper fractions and then subtract, bearing in mind that the LCD of 4 and 2 is 4.
[tex]\bf \stackrel{mixed}{8\frac{3}{4}}\implies \cfrac{8\cdot 4+3}{8}\implies \stackrel{improper}{\cfrac{35}{4}}~\hfill \stackrel{mixed}{7\frac{1}{2}}\implies \cfrac{7\cdot 2+1}{2}\implies \stackrel{improper}{\cfrac{15}{2}} \\\\[-0.35em] ~\dotfill\\\\ \cfrac{35}{4}-\cfrac{15}{2}\implies \stackrel{\textit{using the LCD of 4}}{\cfrac{(1)35~~-~~(2)15}{4}}\implies \cfrac{35-30}{4}\implies \cfrac{5}{4}[/tex]
