If three points are on the same line segment, than each of the point should have the same slope with either of the other.
Slope of the points from the stem:
so let's try the answers. Take (5,6) because the numbers will be lower and thus easier to calculate with:
A) (9-6)/(9-5)=3/4 wow, that seems like our candidate...
Lets check B as well, just to be sure
B) (12-6)/(12-5)=6/7 ... different slope so the point will not be on that linesegment.
there is no need to calculate any further, so we save time