It appears that you added the coefficients of the first terms instead of squaring them. 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\$ . In this case $\displaystyle{a}={5}{d}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={4}\usepackage{cmbright}\Huge\$ .