4. 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: Month Units Shipped 30 Total Shipping Expense $ 1,800 January February 60 $ 2,300 March 40 $ 1,700 50 April May $ 2,000 $ 2,300 70 June 80 $ 2,700 July 20 $ 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)