Substitute 64 for A in the formula and solve: $\displaystyle{64}={{s}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle{{s}}^{{2}}-{64}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{\left({s}-{8}\right)}{\left({s}+{8}\right)}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{s}-{8}={0}\usepackage{cmbright}\Huge\$ or $\displaystyle{s}+{8}={0}\usepackage{cmbright}\Huge\$

$\displaystyle{s}={8}\usepackage{cmbright}\Huge\$ or $\displaystyle{s}=-{8}\usepackage{cmbright}\Huge\$

The length of the side is 8, because lengths cannot be negative.