y = -x + 2x ⇒ y = x
I can't graph it here but I'll tell you exactly how to do it in the shortest answer possible :)
Start at (0, 0) and go up with a slope of 1.
That means you go up 1 unit and go right 1 unit.
You should be at (1, 1).
Connect the points using a straightedge and you should have a line that goes through the points (0, 0), (1, 1), (2, 2), (3, 3), (4, 4), (5, 5), and so on...
Hope this helped!