I don't understand what this question

Just desserts buys cheesecakes from general bakeries at $33.60 per cheesecake. It then cuts each cheesecake into 16 slices and sells them to customers at $6.50 per slice. Calculate the rate of markup on cost and the rate of markup on selling price.

I run into trouble when i try to calculate the markup on cost because the formula is MarkUP = Selling price - Cost