The rate of change is rate of how y changes in time in terms of the rate of change of x in time. It can be found by deriving the function once and applying chain rule:
If they paid $0.72 in tax and tax is 6% or .06 then you could create an equation like this: x * .06 = .72 with x representing the original cost of the bracelet. Now you can just solve for x like so: