Correct. The sum of all the side lengths equals the perimeter.

$\displaystyle{5}+{6}+{4}+{6}+{x}={25}\usepackage{cmbright}\Huge\$

$\displaystyle{21}+{x}={25}\usepackage{cmbright}\Huge\$

$\displaystyle{x}={4}\usepackage{cmbright}\Huge\$