Pythagorean theorem

The hypotenuse is the side opposite the right angle, and always the longest.

Here the hypotenuse is the missing one, so the two given sides are both legs: 3 and 4.

Square the two you have: 3² = 9 and 4² = 16.

The two squares ADD to the square on the hypotenuse: 9 + 16 = 25.

Now take the square root: √25 = 5 cm.