72 is between the perfect squares 64 and 81. So $\displaystyle\sqrt{{{72}}}\usepackage{cmbright}\Huge\$ is between $\displaystyle\sqrt{{{64}}}\usepackage{cmbright}\Huge\$ and $\displaystyle\sqrt{{{81}}}\usepackage{cmbright}\Huge\$ .

$\displaystyle\sqrt{{{64}}}={8}\usepackage{cmbright}\Huge\$ and $\displaystyle\sqrt{{{81}}}={9}\usepackage{cmbright}\Huge\$

Because 72 is closer to 64 than it is to 81, $\displaystyle\sqrt{{{72}}}\usepackage{cmbright}\Huge\$ is probably about 8.4 or 8.5.

$\displaystyle{8.4}\cdot{8.4}={70.56}\usepackage{cmbright}\Huge\$ and $\displaystyle{8.5}\cdot{8.5}={72.25}\usepackage{cmbright}\Huge\$

72 is closer to 72.25 than 70.56, so the answer is 8.5.