Phillips Company manufactures air-conditioning units for commercial buildings and has noticed considerable variation in shipping expenses from month to month as per the data below: Month Units Shipped Total Shipping Expense January 4 $ 2,120 February 4 $ 3,020 March 5 $ 2,500 April 2 $ 1,278 May 3 $ 2,130 June 6 $ 2,880 July 8 $ 4,128 4. What is the cost formula for shipping expenses