Correct. $\displaystyle{24}{x}={2}\cdot{2}\cdot{2}\cdot{3}\cdot{x}\usepackage{cmbright}\Huge\$ and $\displaystyle{36}{{x}}^{{2}}={2}\cdot{2}\cdot{3}\cdot{3}\cdot{x}\cdot{x}\usepackage{cmbright}\Huge\$ , so the LCM $\displaystyle={2}\cdot{2}\cdot{2}\cdot{3}\cdot{3}\cdot{x}\cdot{x}={72}{{x}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle\frac{{{7}{b}}}{{{24}{x}}}-\frac{{{7}{b}}}{{{36}{{x}}^{{2}}}}=\frac{{{7}{b}{\left({3}{x}\right)}}}{{{24}{x}{\left({3}{x}\right)}}}-\frac{{{7}{b}{\left({2}\right)}}}{{{36}{{x}}^{{2}}{\left({2}\right)}}}=\frac{{{21}{b}{x}}}{{{72}{{x}}^{{2}}}}-\frac{{{14}{b}}}{{{72}{{x}}^{{2}}}}=\frac{{{21}{b}{x}-{14}{b}}}{{{72}{{x}}^{{2}}}}=\frac{{{7}{b}{\left({3}{x}-{2}\right)}}}{{{72}{{x}}^{{2}}}}\usepackage{cmbright}\Huge\$