If x is the length of the first side, then $\displaystyle{x}+{1}\usepackage{cmbright}\Huge\$ will be the length of the second side, and $\displaystyle{x}+{2}\usepackage{cmbright}\Huge\$ will be the length of the third side. The perimeter is the sum of the three sides. So $\displaystyle{x}+{\left({x}+{1}\right)}+{\left({x}+{2}\right)}={63}\usepackage{cmbright}\Huge\$ .

Combine like terms: $\displaystyle{3}{x}+{3}={63}\usepackage{cmbright}\Huge\$

Subtract 3 from both sides and divide by 3: $\displaystyle{x}={20}\usepackage{cmbright}\Huge\$

The third side will be $\displaystyle{x}+{2}={22}\usepackage{cmbright}\Huge\$ feet.