I am preparing for my placement test for college, and I'm stuck on this perimeter problem (it's been awhile since high school):

Two sides of a triangle must be 8 m and 12 m. The perimeter must be at least 24 m, at most 39 m,. determine the length that can be used for the third side.

If you get it, please explain it to me...I feel like there isn't enough information to figure it.