When applying the distributive property, it appears that you made a mistake with a sign. You should have gotten $\displaystyle{\left(-{3}\right)}{\left(-{6}{x}{{y}}^{{2}}\right)}={18}{x}{{y}}^{{2}}\usepackage{cmbright}\Huge\$ , not $\displaystyle-{18}{x}{{y}}^{{2}}\usepackage{cmbright}\Huge\$ .