Attached solution with detailed work.
 
        
        
        
Well a half cup is 2/4 and if the serving size is 3/4 there is only 66.66% of the serving size needed hope this helps!
        
             
        
        
        
Y=-1/3x+4
x2-x1/ y2-y1
(0,4)(2,-2)
2-0/-2-4 
1/-3
y-intercept is 4
y=-1/3x+4
        
             
        
        
        
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