find the length of x in each triangle 12,x,8?

Answer 1

If that's the only information you are given, then the best you can do is to specify a range of values for x. The sum of two sides of a triangle can't be larger than the third side, so for these sides that you have listed, x can't be larger than 20, and it can't be smaller than 4. So 4
If this were a right triangle, you can use the Pythagorean Theorem to figure out what the missing side is, but you'd have to specify which one of these is the hypotenuse in order to use the formula correctly.