Two things:
1. Since you're remembering the question form a test, you might not be recalling the question exactly as it was posed - there could be additional data ..... (Eg. the question might have asked for the simplest parabola - see below)
2. Assuming the square to have centre at (0, 0) [that is, the origin] leads to the simplest equation. But .... note that I've stressed the word assuming .....
I'd suggest you approach whoever set the test and discuss with them the issue raised in this thread.