Hey! So, in doing this, we need to follow the rule for the 3rd angle of a triangle that x has to be greater than 0. So, to solve this you need to do the equation x = perimeter-side-side. We only know the 2 sides, so plugging in the numbers, you get x = perimeter - 5 - 8. Doing this, if we set the perimeter to anything below 17, then x will be lower than 0. So, by setting the perimeter to 17, we get that x can equal 4. Now, if we set the perimeter to the highest it can be for x > 0 to be true, we get 12. So, putting that into an expression, we get x > or = to 4 and x < or = to 12. Thanks!