\[\begin{array}{l}
\frac{3}{4} + \frac{9}{{24}} = \frac{{18}}{{24}} + \frac{9}{{24}} = \frac{{27}}{{24}} = \frac{9}{8}\\
\frac{5}{7} \times \frac{{14}}{{35}} = \frac{{5 \times 14}}{{7 \times 35}} = \frac{{5 \times 2 \times 7}}{{7 \times 5 \times 7}} = \frac{2}{7}\\
\frac{4}{3} - \frac{{12}}{{21}} = \frac{{28}}{{21}} - \frac{{12}}{{21}} = \frac{{16}}{{21}}\\
\frac{7}{9}:\frac{{11}}{8} = \frac{7}{9} \times \frac{8}{{11}} = \frac{{56}}{{99}}
\end{array}\]

$\frac{3}{4}+\frac{9}{24}= \frac{18+9}{24}=\frac{27}{24}=\frac{9}{8}$ 

$\frac{5}{7}x\frac{14}{35}= \frac{2}{7}$

$\frac{4}{3} - \frac{12}{21}= \frac{28-12}{21}=\frac{16}{21}$

$\frac{7}{9}:\frac{11}{8}=\frac{7}{9}x\frac{8}{11}=\frac{56}{99}$

.