Question

Which of the following ordered pairs represents the solution to the system given below?

Answer 1

x − 3y = 12
2x + 6y = 60

x=12+3y

2(12+3y)+6y=60

24+6y+6y=60

24+12y=60

12y=60-24

12y=36

y=36/12

y=3

and since the only answer you have with y being 3 is 21,3 the answer is D.(21,3)

Answer 2

{ x - 3 y = 12  *  ( - 2)
  2 x + 6 y = 60 

- 2 x + 6 y = - 24
  2 x + 6 y = 60
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          12 y =  36
                 
               y = 36 / 12

               y =  3

For x :

2 x + 6 y = 60

2 x + 6 * ( 3 ) = 60

2 x + 18 = 60

2 x = 60 - 18

2 x =  42

x = 42 / 2

x =  21

Answer D : ( 21,3)  

hope this helps!