Add the terms straight down (ie add like terms)
For the x terms
(3x) plus (-3x) = 3x-3x = (3-3)x = 0x = 0
The result being zero means the x terms effectively cancel out and go away
For the y terms
(-y) plus (3y) = -1y+3y = (-1+3)y = 2y
And finally the terms on the right hand side
(-1) plus (27) = -1+27 = 26
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After adding the two equations, we end up with 0+2y = 26
which simplifies to 2y = 26. So the answer is choice D
