The Pythagorean Theorem: a² + b² = c²
a and b is the other sides, while c is the hypotenuse (or largest side)
The Pythagorean Theorem is used when you are trying to find one of the sides, and you know the length of the other two sides.
A real life example can be people trying to build a bridge, finding the right size computer for a table, etc
hope this helps