I am slightly confused on the following problem:

Change the order of integration of:

by making the substitution u = x, v = 2y-x.

Doing so I get:

However...it should apparently become:

That's where I am confused. I was able to see how to change the limits of integration, but I don't know where this 1/2 comes from. Consequently my answer is twice as much as it should be. Thanks for any help.