Answer:
x3 = y
Step-by-step explanation:
This is assuming x represents the number of notebooks, as it isn't stated. The store sells notebooks for 3 each so one notebook will cost 3, two will cost 6, etc. The total cost is based on how many notebooks were bought and the cost, and they are multiplied together. As y represents the total cost of the notebooks, the equation is x3 = y (or y = x3, since where the equals sign doesn't matter).
Hope this helps !!