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: Month Units ShippedTotal Shipping Expense January February6 $ 1,800 $ 2,300 $ 1,700 $2,000 2,300 $ 2.700 $ 1,200 4 March April May June July Required: Using the high-low method, calculate the equation of a cost line for the shipping expense for this company (Y- a+ bX)