The pythagorean theorem was originally put forward by pythagoras as a way of working out the lengths of the sides of a triangle if you have two of the sides already and need the last side. This only works on a right angled triangle, however. The formula he put forward was a^2 + b^2 = c^2, where c is always the hypoteneuse of the triangle and the other two (a and b) are the adjacent and opposite. Squaring these numbers in the formula will allow you to work out the missing side, although it may need to be rearranged to c^2 - a^2 = b^2 or c^2 - b^2 = a^2 to work out the adjacent and opposite.
