The triangle inequality theorem states that when you add any 2 sides of a triangle together they have to be greater than the 3rd side. 5 is the lowest value we can use to keep this triangle a triangle because 8+5 = 13 which is just one unit greater than 12. The highest we could feasibly go to make the triangle just one unit away from not being a triangle anymore would be 19 because 8+12 = 20 which is just one number greater than 19. So your inequality is this:
5<x<19 with the equals sign underneath each of those < (read: x is greater than or equal to 5 and less than or equal to 19)