Here's a worked example I don't understand:

Use Green's Theorem to evaluate

along the triangular path with verticies at: (0,0), (1,0) and (1,2).

Answer:

Now, I don't understand how they got " " as the upper limit for the integral in:

??

I did sketch the region. So, is "2x" the equation of the line from (0,0) to (1,2)? If so, how did they work out this equation for it?