We'll solve the system:
by elimination by addition.To eliminate by addition, we need to set both coefficients of x to numbers with changed signs, i.e a and -a. Since in the second equation we have 1 as our coefficient for x, to get -3 we have to multiply all terms of the second equation by  which is equal to -3.
Multiplying, we get on our second equation:
Adding both equations we get:
Since 3 and -3 cancel out, we have a linear equation:Therefore, we know that y = 6.
Plugging that in into the first equation gives us:
Therefore, our answer is:
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