Pythagoras theorem helps to find the hypotenuse of especially a right angled triangle. Especially because you can use the theorem to find the hypotenuse of an equilateral triangle if it is divided into two right angled triangles.
There are 3 sides of a right angled triangle, the opposite, the adjacent(base) and the hypotenuse (longest side). If you are given the measurements of 2 sides you can find the third. if you are given the opposite and the adjacent , the formula for finding the hypotenuse is a²+b²=c², where a is the opposite, b is the adjacent and c is the hypotenuse.