The sales tax in one state is 6 percent, write a function rule for finding the total cost for an item with selling price x, then find the selling cost of a cd player with the selling price of $160 f(x) =

Sales tax = 6% or 6/100. 
assuming tax is added onto the item:
total cost function f(x) = price + tax
however, assuming the cost price = sales price  - tax,  then:
f(x) = sales price - tax 
This is not really a business or accounting forum, its a maths forum, so you need to be clearer about business or accounting terms such as total cost, selling price, selling cost.

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