Arron just paid $7.40 for a new pair of sunglasses.
The price of pair of sunglasses ten years ago was $6.80
The price of pair of sunglasses twenty years ago was $6.20
To get the price of pair of sunglasses a decade ago:
You take the current price which is $7.40,
you subtract ($7.40 - $6.80) = ($6.80 - $6.20) = $0.60
*NB : The common difference here is $0.60
So the price(P) of pair of sunglasses x decades ago is = $7.40 - $0.60x
P = $7.40 - $0.60x
Plotting the graph of Price(P) of a pair of sunglasses against number of decades (x) should be easy now right?