It seems that you made a mistake in your first step, trying to isolate the term $\displaystyle{3}{Q}\usepackage{cmbright}\Huge\$ . You cannot do this by subtracting $\displaystyle{2}{P}\usepackage{cmbright}\Huge\$ . You need to add $\displaystyle{2}{P}\usepackage{cmbright}\Huge\$ to both sides.