If two of the four expressions x + y , x +…

If two of the four expressions x + y , x + 7y , x - y , and 7x - y are chosen at random, what is the probability that their product will be of the form of x^2-(by)^2, where b is an integer?

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #39922