Question 14313
If the points are taken as P with coordinates (p,q) and A with coordinates (a,b)
then the slope of the line PA is given by the formula 
   Difference in Y coordinates / Difference in  X coordinates 
   = (b-q)/(a-p)...It does not matter for this purpose whether we take the coordinates of A first or those of P first as long as we take the same order for calculating the difference of  both Y and X cordinates of the 2 points.It is
easy to see that 
  Slope of PA = (b-q)/(a-p) = (-(q-b))/(-(p-a))= (q-b)/(a-p)= Slope of AP
   Viewing it from the physical sence , slope of a line is the Tangent of the angle the line makes with X axis in the positive (anticlockwise)direction. The line afterall makes the same angle with X axis , whether we look at it as PA or AP and hence its tangent which is the slope of the line is also same whichever way we take the line segment.