Your cart will need to have a column for category to determine whether the discount should be applied or not.
this calculation assumes the category is set to "1" in the add to cart button for golf clubs:
Set the TotalPrice calculation to:
(([Price] * [Quantity]) - (([Quantity]>6 && [category]=='1')?(([Quantity]-6)*18):0))
if you want to display the savings, create an new calculation called savings:
([Price] * [Quantity]) - (([Price] * [Quantity]) - (([Quantity]>6 && [catagory]=='1')?(([Quantity]-6)*18):0))
When you create your cart display page, on the second page off the disp[lay manager, you can select columns to show, click the plus button and select the saving calculation to show it in the cart display.