From what you say, using the US gallon, since both answers round to the same number (since there is no currency below the cent), it wouldn't matter too much. Also, unit conversions should not lead to different answers.
Perhaps the imperial value which would have been 4.50 per gallon which is an imperial gallon (used in Canada). It's hard to tell. I would have thought 3.78 should be more than close enough.
r is the x coordinate and s is the y coordinate. Positive slopes go from lower left to upper right, like going up stairs or up a hill. The line that goes through (r, s) that is positive is line B.
