I assume you're referring to the Pythagoras theorem, the rule states that In a right triangle, to determine the length of the hypotenuse (or legs if you fit the equation), you must gather the length of both legs (a and b) and square them. The final number once you add the squared numbers together should be the length of the hypotenuse squared, so a²+ b²=c². And if you wish to determine the length of one of the legs, you need the length of the hypotenuse and the other leg, the equation for this will be: a²(or b²)-c²=b²(or a²).