Explanation:
Slope formula:
(y2-y1) is the rise and (x1-x2) is the run which accumulates to form the gradient: rise/run. This slope equation id denoted as (variable 1 and 2) since there are 2 x and y values given to mathematically find the gradient. So there is no order in which you substitute your values to find the slope.
Due to formatting error I cannot comprehend the assignements of coodinates in your previous question.