Imagine the 3 and 8 as being the lengths of two sides of a triangle. The third side's length would be x. If the angle formed is 0 or 180 degrees, the figure won't be a triangle, so it has to be between these.

When the angle is 0, the two lines are on top of each other, and the length of the third "side" will be their difference, 5. So x must be greater than 5.

If the angle is 180 degrees, the two given lines would be end-to-end, and a line connecting them would have the length of their sum, 11. So x must be less that 11.