$$( \frac { 1 } { 2 a } - \frac { 1 } { 3 b } ) ^ { 2 }$$
$\frac{\left(\frac{3b-2a}{ab}\right)^{2}}{36}$
$\frac{4a^{2}-12ab+9b^{2}}{36\left(ab\right)^{2}}$
