To find the third side you would use Pythagorean theorem which is a²+b²=c².
A and B being the 2 legs and C being the hypotenuse.
Example: ( sorry for the really odd not really a triangle, triangle...)
| \
6| \ ? 6²+9²=C²
|_______ 36+81=C²
9 117=C²
√117=10.8 (rounded)
if you need to get the leg you just fill in the numbers.