So you BEGIN with 20, which is your membership fee (it only gets charged once) then your mx is your slope. 2.50 is your slope because they earn 2.50 for each (x) video rented.
you place a number beside the x for each video rented. For example, if you rent 2 videos then you write this y=2.50(2)+20
3.27 - 2.64 = 0.63, but for scientific notation, you need a number that's larger than 1 as the multiplier, so we change it to 6.3 and 0.63 but chang *-7 to *-8