You did not apply the distributive property. First, you should have multiplied $\displaystyle-{6}{x}{{y}}^{{2}}\usepackage{cmbright}\Huge\$ by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ . Second, you should have multiplied $\displaystyle-{7}{x}{y}\usepackage{cmbright}\Huge\$ by $\displaystyle{4}{y}\usepackage{cmbright}\Huge\$ . This would give you two like terms that should be combined.