Answer:
Hi! I think you are referring to the Pythagorean Theorem.
Essentially, given a right triangle, the sum of the squares of the shorter sides equals the square of the longer (slant) side or hypotenuse.
The formula is a squared + b squared = c squared
If the triangle has side lengths 3 and 4 (for the shorter sides), the longer side can be solved like this:
3 squared + 4 squared = x squared
(Let x = the longer side)
3 squared = 9
4 squared = 16
9+16 = x squared
25 = x squared
x = 5
So there you go, that's the Pythagorean theorem.
Let me know if this helped!