The Pythagorean Theorem is a² + b² = c², where c is the hypotenuse of a right triangle. The hypotenuse of the triangle is always opposite the right angle; in this case, the hypotenuse measures 5 units. To find the missing side length, plug the given values into the theorem and solve algebraically for x.
x² + 4² = 5² (plug in values for hypotenuse and side length) x² + 16 = 25 (square 4 and 5) x² = 9 (subtract 16 from both sides) x = 3 (take the square root of both sides)