It appears that you tried to factor out the common factor $\displaystyle{\left({x}+{3}\right)}\usepackage{cmbright}\Huge\$ from $\displaystyle{x}{\left({x}+{3}\right)}+{4}{\left({x}+{3}\right)}\usepackage{cmbright}\Huge\$ , but made a mistake with the result. 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\$ . In this case, $\displaystyle{b}={x}\usepackage{cmbright}\Huge\$ , $\displaystyle{c}={4}\usepackage{cmbright}\Huge\$ , and $\displaystyle{a}={\left({x}+{3}\right)}\usepackage{cmbright}\Huge\$ .