You may have simplified the radicals correctly, obtaining $\displaystyle{{x}}^{{3}}{y}-{2}{{x}}^{{3}}{y}\usepackage{cmbright}\Huge\$ , but then tried to cancel a factor of $\displaystyle{{x}}^{{3}}{y}\usepackage{cmbright}\Huge\$ in the second term with the first term. You combine like terms by adding or subtracting their coefficients, not by canceling their variables, $\displaystyle{a}-{2}{a}={1}{a}-{2}{a}={\left({1}-{2}\right)}{a}=-{1}{a}=-{a}\usepackage{cmbright}\Huge\$ .