Let R2 denote the usual (x, y)-plane. We can define a partition of R2 by using the

lines with slope 2. Describe the corresponding equivalence relation ∼ by giving the

conditions on the coordinates so that (x1, y1) ∼ (x2, y2).

I have we must have (y2-y1)/(x2-x1)=2

Then y2-y1=2x2-2x1

y2-2x2=y1-2x1

So x1=2x2?