When you used the Distributive Property, you may have added the radicands instead of multiplying them, obtaining $\displaystyle{3}\sqrt{{{9}{x}}}+\sqrt{{{8}{x}}}\usepackage{cmbright}\Huge\$ , and then simplified and combined. Remember that $\displaystyle\sqrt{{a}}\cdot\sqrt{{b}}=\sqrt{{{a}{b}}}\usepackage{cmbright}\Huge\$ .