You multiplied only the first terms and the last terms, and you made a mistake with signs. Remember that a negative times a negative is a positive. To get the part that is missing, multiply the outer terms, $\displaystyle{\left({2}{x}\right)}{\left(-{2}{y}\right)}\usepackage{cmbright}\Huge\$ , multiply the inner terms, $\displaystyle{\left(-{3}{y}\right)}{\left({6}{x}\right)}\usepackage{cmbright}\Huge\$ , and add these to the terms you have.