Substitute 144 for A in the formula and solve: $\displaystyle{144}={{s}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{{s}}^{{2}}-{144}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{\left({s}-{12}\right)}{\left({s}+{12}\right)}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{s}-{12}={0}\usepackage{cmbright}\Huge\$ or $\displaystyle{s}+{12}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{s}={12}\usepackage{cmbright}\Huge\$ or $\displaystyle{s}=-{12}\usepackage{cmbright}\Huge\$

The length of the side is 12, because lengths cannot be negative.