4. (10 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: Total Shipping Expense Units Shipped 30 60 40 Month January February March April May June July 50 $ 1,800 $ 2,300 $ 1,700 $ 2,000 $ 2,300 $ 2,700 $ 1,200 70 80 20 Required: Using the high-low method, calculate the equation of a cost line for the shipping expense for this company (Y= a +bX)