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Shell gas station is having a sale on diesel. If you buy `6` gallons or less, you pay the regular price of `$3.59` per gallon. If you buy more than `6` gallons, you pay `$3.39` per gallon. Write a piecewise function for the price of `n` gallons of diesel at Shell.

` D(n)= {( ),( ),( ):}`   if `0<n ≤`
  if `n>`