Assuming it doesn't have to be a right triangle, you have a whole range of options. Only limitation is that any side of a triangle has to be less than the sum of the lengths of the other two sides. So, on the long end, it has to be shorter than the sum of the first two (i.e. <8). However, on the short end it has to be longer than the difference of the other two sides (i.e. >2). Otherwise, the side with length 5 would break the rule. Therefore, we can choose a third length L that is in 2 < L < 8. Note that those are strict inequalities. If we actually chose 2 or 8, we'd have a straight line, not a triangle.
If you have any other question, let me know.
