We want to maximize C, which is C= 7x - 3y. Let's explore our intuition regarding how this will look. We know that both y and x are greater than or equal to zero, which means -3y will be a negative number (or 0) and x is a positive number (or 0). We want to get x as big as possible, given the constraints, we want y to be zero, if possible
We see that, based on the constraints, y is greater than or equal to zero, but it has no other minimum value established. We only know that y and x add up to 5, and that y is less than or equal to (1/5)x + 2. That means y could be 0 and still satisfy the constraints.
In order to maximize C, we want to be 5 as possible and y to be zero:
C = 7 (5) - 3(0)
C=35