You may have incorrectly simplified by adding the radicands, obtaining $\displaystyle\sqrt{{{28}}}-\sqrt{{{16}}}\usepackage{cmbright}\Huge\$ . Remember that $\displaystyle\sqrt{{a}}\cdot\sqrt{{b}}=\sqrt{{{a}{b}}}\usepackage{cmbright}\Huge\$ . After you perform this multiplication, simplify each radical.