It appears that you added the two common factors together: $\displaystyle{\left({x}+{5}\right)}+{\left({x}+{5}\right)}={2}{x}+{10}\usepackage{cmbright}\Huge\$

Remember that you are really using the distributive property in reverse: $\displaystyle{b}\cdot{a}+{c}\cdot{a}={\left({b}+{c}\right)}{a}\usepackage{cmbright}\Huge\$ . Notice that the common factor a does not end up doubled. In this case, $\displaystyle{b}={x}\usepackage{cmbright}\Huge\$ , $\displaystyle{c}={6}\usepackage{cmbright}\Huge\$ , and $\displaystyle{a}={\left({x}+{5}\right)}\usepackage{cmbright}\Huge\$ .