Here's the question you clicked on:
Geometry_Hater
Determine whether each equation defines y as a function of x x + y = 25
Hmm, they're probably asking, is this equation written explicitly in terms of x. Example: y=25-x This is the same equation as the one you have written, but with the x moved so its written so that y is a function of x. The way they wrote yours, it is not explicitly written as y a function of x. At least i think that's what they're asking :)