The Pythagorean Theorem is one of the most important formulas used in geometry and mathematics. It is important to understand how and why the Pythagorean Theorem works in order to understand how to use it.
Once you understand the Pythagorean Theorem it will be a piece of cake finding sides of right triangles. This concept won't disappear either, the Pythagorean Theorem will follow you into advanced math concepts and even other areas such as physics and biomechanics.
In a right angled triangle:
the square of the hypotenuse is equal to
the sum of the squares of the other two sides.
This a cool video showing visually how the Pythagorean Theorem works.
Watch a quick description of the Pythagorean theorem and how to solve with it.