Thanks for your question!
Upon a quick look, it seems that the Price is charged monthly, whereas the Cost is calculated per rental. As the Number of rentals per month is 6, you need to multiple the Cost by 6 to get to the Monthly Cost. So your monthly Profit is your Monthly Price minus Monthly Cost, hence:
Margin = (Price (monthly) - Cost (monthly)) / Price (monthly) = (Price - Cost per rental * Number of rentals) / Price = (Price - $1.75*6) / Price
Hope this helps!