I need help with solving for b in slope intercept from two points

I have no idea if this is the proper place to ask this question, but I'll give it a try.  I am running into trouble on the practice quiz for finding slope intercept from two points.  I can always find the correct slope, that's simple, but I do not know which ordered pair to use to plug in for finding b.  The lesson video implies that one can use either to get a correct answer, but this is not the case.  I always use the second pair, and am often wrong.  When I click on the "get help" tab to see an explanation of my error I see that the first ordered pair has been used.  This seems to be totally arbitrary and I am very frustrated at this point.  The answer is sometimes found using the first ordered pair, sometimes the second, with no discernable pattern.  I would like to know how to choose the correct ordered pair.