Think of two perfect squares that surround 45. The number 45 is between the perfect squares 36 and 49. So $\displaystyle\sqrt{{{45}}}\usepackage{cmbright}\Huge\$ must be between $\displaystyle\sqrt{{{36}}}\usepackage{cmbright}\Huge\$ and $\displaystyle\sqrt{{{49}}}\usepackage{cmbright}\Huge\$ .

$\displaystyle\sqrt{{{36}}}={6}\usepackage{cmbright}\Huge\$ and $\displaystyle\sqrt{{{49}}}={7}\usepackage{cmbright}\Huge\$ , so $\displaystyle\sqrt{{{45}}}\usepackage{cmbright}\Huge\$ falls between 6 and 7.