B2-4ac
When:
a=2
b=5
C=0

10

Step-by-step explanation:

5*2-4*2*0

10-0

10

X > 0 , everything to the right of the y-axis 
y > 0 , everything above the x-axis 
So far you are in quadrant I of the x-y plane
y<1/5x+2
sketch a line leaning 45° to the left with a y-intercept of 2, shade in everything below that line 
now our Objective Function is 
C = 7x - 3y , which is a line with slope 7/3 
letting that line go through the origin, let it slide parallel to itself over the shaded region until you reach the farthest point from the origin. 
Here are the required values : 
c = 35; x = 5, y = 0