It appears that you did not distribute the $\displaystyle-{1}\usepackage{cmbright}\Huge\$ to all terms in the trinomial. Always multiply each term in the first polynomial with each term in the second polynomial. The correct answer is $\displaystyle{6}{{y}}^{{3}}+{13}{{y}}^{{2}}+{16}{y}-{7}\usepackage{cmbright}\Huge\$ .