The Pythagorean Theorem is a² + b² = c².
You use this formula to find a missing side on a right triangle. a and b are the sides that make up the right angle, and c is always the longest side of the triangle, so the one directly opposite of the right angle. When you have a and b, you square them, add them together, and find the square root of the sum to find c.
When you have a OR b and c, you square them both, and then divide c by a OR b to find the missing side. The Pythagorean Theorem is extremely useful.
Hope that helps!