Exponential function two point formula doesn't make sense

Hello, I have a test soon, and in one question I am to proof the two point formula for exponential functions.

I don't understand why you divide y2/y1. If you divide the other way, y1/y2, you get a formula that doesn't work. I understand you do it to isolate a, but why does it have to be y2/y1.

All the videos I watched or books doesn't explain why you divide y2/y1 and not y1/y2. Can anyone explain? Is it just some kind of cheatcode? Thanks.