The Pythagorean theorem
In a right triangle, the two shorter sides squared add up to the longest side squared.
A worked example
A ladder 5.0 m long has its foot 1.4 m from a wall. It reaches √(5.0² − 1.4²) = √23.04 = 4.8 m up the wall.
The mistake people actually make
The hypotenuse is always the longest side and always sits opposite the right angle. Check you have not squared the wrong side.