Question 120769
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.
{{{a^2+b^2=c^2}}}
where the sum of the squares of the shorter sides equals the square of the largest leg. 
{{{50^2+80^2=c^2}}}
{{{c^2=2500+6400}}}
{{{c^2=8900}}}
{{{c=94.3}}}
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).