4. (4 points) Archer Company is a wholesaler of custom-built air-conditioning units for commercial buildings. It gathered the following monthly data relating to units shipped and total shipping expense MonthUnits ShippedTotal Shipping Expense January February March April May June July 30 60 40 50 70 80 20 $ 1,800 $ 2,300 $ 1,700 $ 2,000 $ 2,300 $ 2,700 $ 1,200 Required: Using the high-low method, calculate the equation of a cost line for the shipping expense for this company (Y -a + bX)