So if 13 km is the longest side, then the empty side has to be in the middle of the 2, which the only answer that fits it is 12.
Or you can do it the original way.
for the equation, you would have to bring c squared, which is 13, over to the side with 5 so the blank side is by itself.
so then square 13 and 5
13 squared is 169
5 squared is 25
subtract
169-25=144
144's square root is 12
so there is your answer is 12 km.