First, multiply the first equation by 2.
That turns the first equation into:
6x - 2y = 18
That way it can easily be added to the second equation:
6x - 2y = 16
x + 2y = 5
---------------
7x = 21
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As you can see, the y disappeared, which was the objective of this operation.
Now the equations can be solved easily:
7x = 21
x = 3
Insert x=3 in the original equation:
3*3 - y = 8
9 - y = 8
9 = 8 + y
y = 1
So the result is:
x = 3
y = 1