Here's the question you clicked on:
DHAT
Section 12 - Distance to Planes Problem 1 - Why is vector PQ = <1,0,0>? Shouldn't it be <-1,0,0> since we are subtracting P from Q?
You are right. However, that doesn't make a difference in this case, since the other two coordinates are cero (the dot product gives you the same number with opposite sign), and then you take the absolute value at the end.