If you are given a line on a graph and asked to determine the slope, does it matter which coordinates you chose?
When trying to figure the slope from your own choice of coordinates on a line, does it matter when coordinates you choose?
Any choice of points on a line will give you the same result when you calculate the slope. For lines slope is a constant, and so will not be dependent on which points you choose to "plug into" the formula. Hope this answers your question, and if there is anything else you need assistance with, I or another tutor will be happy to help.