Think of W as the variable and L as the coefficient. To isolate W, divide both sides by L:

$\displaystyle\frac{{A}}{{L}}=\frac{{{L}\cdot{W}}}{{L}}\usepackage{cmbright}\Huge\$ or $\displaystyle\frac{{A}}{{L}}=\frac{{L}}{{L}}\cdot{W}\usepackage{cmbright}\Huge\$ or $\displaystyle\frac{{A}}{{L}}={W}\usepackage{cmbright}\Huge\$