Substitute 42 for A and 7 for b: $\displaystyle{42}=\frac{{1}}{{2}}\cdot{7}\cdot{h}\usepackage{cmbright}\Huge\$

Multiply both sides by 2: $\displaystyle{84}={7}\cdot{h}\usepackage{cmbright}\Huge\$

Divide both sides by 7: $\displaystyle{12}={h}\usepackage{cmbright}\Huge\$