SOLUTION: My husband is needing to know what size mat to get based on the answer to this question - if the bottom side is 50', the second side is 80', what would the third equal? This, of c

You can put this solution on YOUR website!
The answer depends on the angles involved with the triangle.
If it's a right triangle, that is, one of the angles is 90 degrees, then the answer is straightforward.
Then you could use the Pythagorean theorem.

where the sum of the squares of the shorter sides equals the square of the largest leg.




I assumed that you're looking for the longest side of the triangle.
It was unclear in your description.
If it's not the case that you have a right triangle, it's a little more work.
You would need to re-post with the angle information (at least one angle).