The Pythagorean Theorem says that $\displaystyle{{a}}^{{2}}+{{b}}^{{2}}={{c}}^{{2}}\usepackage{cmbright}\Huge\$ , where a and b are the lengths of the legs of a right triangle, and c is the length of the hypotenuse. In the given triangle, the legs are x and 4, and the hypotenuse (opposite the right angle) is 5.