Spivak Chapter 4 Question 7 I'm a bit confused about this problem, from chapter 4 of Spivak's Calculus.

In particular, I'm not sure what a straight line is defined as in Spivak. Earlier in the text, Spivak defines a straight line as a certain collection of pairs, including, among others, the collections {$(x, cx)$: x a real number}. However, it doesn't seem right to use this definition of a straight line - that straight line is a set of all points such that {$(x, 
-(A/B)x + C)$: x a real number}, as it would make the problem trivial. So what should I treat the definition of a straight line as, for this problem?
Thanks in advance.
Also, could you avoid giving any hints to this problem, as I would still like to attempt solving it myself. Thank you.

A straight line, if it's not vertical can be written in the form of $(x, mx+c)$. If it's vertical, then it can be written as $(x_0, y)$ where $x_0$ is fixed.
