Zerbel Company, a wholesaler of large, custom-built air-conditioning units for commercial buildings, has I noticed considerable fluctuation in its shipping expense from month to month, as shown below: Month Total Shipping Expense Required: January February.. March.. April... May June... July. Units Shipped 4 $2,200 7 $3,100 5 $2,600 2 $1,500 3 $2,200 6 $3,000 8 $3,600 a) Using the high-low method, estimate the total cost formula for shipping expense. b) Using the formula estimated above, what is the total shipping cost will be if the Unites shipped is 12