What side length can make a triangle with side lengths 8 and 12?

This would require use of the Pythagorean Theorem. If you don't know what this is, it's a formula for determining side lengths of triangles. The longest side is the hypotenuse. The other 2 are called the legs. What you do is: you square the side lengths. This means you multiply the length by itself. then you have to make sure the hypotenuse squared is larger than both legs squared.