well, is namely just their sum, let's firstly convert the mixed fractions to "improper" and then add them,
[tex]\bf \stackrel{mixed}{7\frac{2}{3}}\implies \cfrac{7\cdot 3+2}{3}\implies \stackrel{improper}{\cfrac{23}{3}}
\\\\\\
\stackrel{mixed}{9\frac{3}{4}}\implies \cfrac{9\cdot 4+3}{4}\implies \stackrel{improper}{\cfrac{39}{4}}\\\\
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\cfrac{23}{3}+\cfrac{39}{4}\qquad \stackrel{LCD~of~12}{\implies }\qquad \cfrac{4(23)+3(39)}{12}\implies \cfrac{92+117}{12}
\\\\\\
\cfrac{209}{12}\implies 17\frac{5}{12}[/tex]