You correctly factored 28 into $\displaystyle{4}\cdot{7}\usepackage{cmbright}\Huge\$ and took the square root of 4 to get a coefficient of 2 in front of the radical, leaving the simplified factor $\displaystyle\sqrt{{{7}}}\usepackage{cmbright}\Huge\$ . However, you did not simplify the variable factors within the radical expression.