Answer:
68
Step-by-step explanation:
First, we can try to set x as big as possible. To do that, we realize that x<=5, so the biggest value possible of x is 5.
Now, we want to set y as big as possible. To do that, we need to use the fourth equation 4x-y>=1. Since x is 5, the greatest possible value of y is 19.
After checking that x and y fit the constraints, we can plug these 2 numbers in to solve for the biggest value of c;
c=6*5+2*19=30+38=68
