Đáp án:
$\begin{array}{l}
a)\frac{5}{3} = \frac{{5a}}{{3a}}\,\,\,\,\,\,\,\,\,\,\,\,\frac{{5a - 6}}{{3a}}\\
b)\frac{{2x}}{3} = \frac{{2ax}}{{3a}}\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{{2ax + 3x}}{{3a}}\\
c)\frac{b}{{6a}} = \frac{{b.3b}}{{18ab}} = \frac{{3{b^2}}}{{18ab}}\,\,\,\,\,\,\frac{{4a + 3{b^2}}}{{18ab}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{x}{{9b}} = \frac{{x.2a}}{{18ab}} = \frac{{2ax}}{{18ab}}
\end{array}$