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 January February March April May June July Units ShippedTotal Shipping Expense 30 $ 1,800 60 $ 2,300 40 $ 1,700 50 $ 2,000 70 $ 2,300 80 $ 2,700 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)