Pythagoras's Theorem
In any right-angled triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides.
i.e.: c² = a² + b² in the following diagram:

Example:
Find AC in the diagram below.

AB² + AC² = BC²
AC² = BC² - AB²
       = 13² - 5²
       = 169 - 25 = 144
AC  = 12cm