I'm assuming your constraints contain ≤ and ≥, rather than < and >.
The expression (12x-4y) will be at a maximum when x is as large as possible and y is as small as possible. The smallest y can be is 4. Since x+2y≤16, the largest x can be is 8. Therefore, c=12x-4y has its maximum at (8,4), which is c=80.