Thanks, Neil. I structured this exercise based on the problem in my algebra book, which specifically asks for it this way:
Solve for x if y=4x and x+y=25
So for the set of values for y of {4x}, provide x. I'm not all that interested in y, and I really don't want to get the value for it.
Oh, well. Solving for {x,y} does work. I'll keep an eye on it. Thanks, all!