3a+b+c
The perimeter is two times one side (to account for the opposite) and two times the adjacent side. So if the sides would be x and y, the perimeter would be 2*x + 2*y.
So, knowing that the sum is 16a+8b-6c, if we subtract the given side 5a+3b-4c from this, what remains is two times the "other" side:
16a+8b-6c - 2*(5a+3b-4c) =
16a+8b-6c -10a-6b+8c =
6a+2b+2c
half of that is
(6a+2b+2c)/2 = 3a+b+c