Subtract $\displaystyle{3}{Q}\usepackage{cmbright}\Huge\$ from both sides: $\displaystyle{M}-{3}{Q}={2}{P}\usepackage{cmbright}\Huge\$

Divide both sides by 2: $\displaystyle\frac{{{M}-{3}{Q}}}{{2}}=\frac{{{2}{P}}}{{2}}\usepackage{cmbright}\Huge\$

Simplify to: $\displaystyle\frac{{{M}-{3}{Q}}}{{2}}={P}\usepackage{cmbright}\Huge\$