You may have ignored the radical signs or incorrectly thought that they canceled. Rewrite using $\displaystyle\frac{\sqrt{{a}}}{\sqrt{{b}}}=\sqrt{{\frac{{a}}{{b}}}}\usepackage{cmbright}\Huge\$ and simplify. Pull out perfect squares from $\displaystyle{49}{{x}}^{{7}}\usepackage{cmbright}\Huge\$ and simplify, using $\displaystyle\sqrt{{{a}{b}}}=\sqrt{{a}}\cdot\sqrt{{b}}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{7}{{x}}^{{3}}\sqrt{{{x}}}\usepackage{cmbright}\Huge\$ .