Do do this you divide the largest denominator by the smallest;
12/3 = 4
The answer is 4
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The coefficient of x is the value for slope in y-intercept form, and the number that we add or subtract is our y-intercept. So in this case, we want to look for a "-2" in front of x and a +4 in the equation. 
Answer choice A fulfills those conditions. (I am also assuming that answer choice A should have had an x in it, but you accidentally forgot to type it.)
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The irrational number that can be added to pi to get a rational sum is PI.
        
                    
             
        
        
        
Here are the answers:
1) C
2) A
3) D
4) B