Incorrect.You can first use $\displaystyle\sqrt{{a}}\cdot\sqrt{{b}}=\sqrt{{{a}{b}}}\usepackage{cmbright}\Huge\$ to get:

$\displaystyle\sqrt{{{28}}}\cdot\sqrt{{{7}}}=\sqrt{{{28}\cdot{7}}}\usepackage{cmbright}\Huge\$

Rewrite the radicand as a perfect square and take the square root.