You correctly found the least common denominator. However, you probably made a mistake when rewriting the first fraction, replacing $\displaystyle{\left({2}{x}+{1}\right)}{\left({x}+{4}\right)}\usepackage{cmbright}\Huge\$ with $\displaystyle{2}{{x}}^{{2}}+{4}\usepackage{cmbright}\Huge\$ . Do not forget to use FOIL when multiplying binomials.