Subtract and simplify.

$\displaystyle\frac{{{4}{a}}}{{{a}{b}-{b}}}-\frac{{3}}{{{a}{b}}}\usepackage{cmbright}\Huge\$

Domain all a, not $\displaystyle{a}\ne{0}\usepackage{cmbright}\Huge\$ and 1, and all b, not $\displaystyle{b}\ne{0}\usepackage{cmbright}\Huge\$