What is the pythagorean theorem and what is the equation for it?

Hey there! :) 

The Pythagorean Theorem is quite simply a mathematical equation used to determine the third side of a triangle. It can be used to find out ANY side of a given triangle. 

For example, if you are given the hypotenuse of a triangle, as well as another leg, then you can quite simply plug everything in!  

Here is the equation :

a² + b² = c²

Where :

a = random leg of the triangle
b = another random leg of the triangle
c = hypotenuse of the triangle

I've given you an example for future references, placed down below :) 

~I hope this helped!~

Heptagon is the correct ans