Multiply both sides by $\displaystyle\frac{{2}}{{h}}\usepackage{cmbright}\Huge\$ : $\displaystyle\frac{{{2}{A}}}{{h}}={a}+{b}\usepackage{cmbright}\Huge\$

Subtract a from both sides: $\displaystyle\frac{{{2}{A}}}{{h}}-{a}={b}\usepackage{cmbright}\Huge\$