For every value of x, there is a corresponding value of y, and these two values are the ordered pair on that specific point on the graph.
Getting ordered pair is easy as pie. You assign a value to, say, x, then you solve for the y.
x -2y = 6
Say, x=0, so,
0 -2y = 6
y = 6/(-2) = -3
Hence, the ordered pair here is (0,-3).
You need 3 ordered pairs for this equation. You need two more.
say, x=2, then,
2 -2y = 6
-2y = 6 -2 = 4
y = 4/(-2) = -2
Hence, (2,-2) is another ordered pair.
Last one, say x=4,
4 -2y = 6
-2y = 6 -4 = 2
y = 2/(-2) = -1
Hence, (4,-1), your 3rd ordered pair.
Plot those on the same x,y axes, draw a line through them, and you have your graph of the line x -2y = 6.
Now try the second line.