Question

Determine whether each equation defines y as a function of x

x + y = 25

Answer 1

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 :)