let's firstly convert the mixed fractions to improper fractions, and then use the LCD, which in this case is 4, to sum them up.
[tex]\bf \stackrel{mixed}{3\frac{1}{4}}\implies \cfrac{3\cdot 4+1}{4}\implies \stackrel{improper}{\cfrac{13}{4}}~\hfill \stackrel{mixed}{4\frac{3}{4}}\implies \cfrac{4\cdot 4+3}{4}\implies \stackrel{improper}{\cfrac{19}{4}} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ \cfrac{13}{4}+\cfrac{19}{4}\implies \stackrel{\textit{using the LCD of 4}}{\cfrac{13+19}{4}}\implies \cfrac{32}{4}\implies 8[/tex]
