Zerbel Company, a wholesaler of large, custom-built air-conditioning units for commercial buildings, has noticed considerable fluctuation in its shipping expense from month to month, as shown below: Total Shipping Expense $5,100 $6,540 $5,721 $3,420 $ 4,718 $6,403 $11,820 Units MonthShipped January February March 4 ril May June July 19 Required 1. Using the high-low method, estimate the cost formula for shipping expense