While using FOIL, you made a mistake multiplying the last terms. Remember that a negative times a negative is a positive. Also, $\displaystyle-{18}{y}{x}=-{18}{x}{y}\usepackage{cmbright}\Huge\$ , so this and $\displaystyle-{4}{x}{y}\usepackage{cmbright}\Huge\$ are like terms and can be combined.