Answer :

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x+y=2 (label equation one) 
x-y=4 (label equation two)

y=-x+2 (equation one rearranged, label this equation equation three)

sub 3 into 2
x-(-x+2)=4
x+x-2=4
2x-2=4
2x=6
x=3
sub x=3 into equation one

3+y=2
y=-1

therefore x=3 and y=-1