Okay, this is a case of substitution:
3x + y = 13 correct?
-3x -3x
So: y = 13 - 3x
So: 3x +(13-3x)=13
and this is interchangeable, so x and y = all positive numbers
aka. 6x + 2(13-3x) = 26
it does not matter what numbers x and y are as long as they positive