Question

Which is not an equation of the line that passes through the points (1, 1) and (5, 5)?

y - x = 0 <---!
x - y = 0
-x - y = 0

Answer 1

Notice that it's "which is NOT an equation..."

Answer 2

And notice that the first and second are equivalent

Answer 3

And it's therefore the third one which is correct.

Answer 4

Lesson learnt: read your question clearly xDDDDDDDDD (no offense :P)