Use the Pythagorean Theorem, what is the length of the hypotenuse in a right triangle whose legs are 3 and 4?
5 units. This is a very famous triangle.
Then since the side lengths are positive:
The fact that a triangle with sides of 3, 4, and 5 units is a right triangle has been known since at keast the ancient Egyptians. This is the Egyptian triangle, believed to be used by the ancient Egyptians to construct right angles -- for example, in the Pyramids (https://nrich.maths.org/982).