What could be the length of the third side of a triangle that has two sides of lengths 7 and 12?

To create a triangle, the sum of the two shorter sides must be greater than the third side.

If 12 is the longer side then the missing side must be greater than 12 - 7 = 5

If the missing side is the longer side then the missing side must be less than 12 + 7 = 19

Thus any length that is greater than 5 and less than 19.