You may have been thinking of the square of a binomial. To find this type of product, you can use $\displaystyle{\left({a}-{b}\right)}{\left({a}+{b}\right)}={{a}}^{{2}}-{{b}}^{{2}}\usepackage{cmbright}\Huge\$ . Note that there is no middle term in the result. In this case $\displaystyle{a}={6}{x}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={5}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{36}{{x}}^{{2}}-{25}\usepackage{cmbright}\Huge\$ .