The following data relating to units shipped and total shipping expense have been assembled by Archer Company, a wholesaler of large, custom-built air-conditioning units for commercial buildings:

Month Shipped Units Shipped Expense

January ................... 3 ................$1,800 February ................. 6 ................$2,300 March ...................... 4 ................$1,700 April ........................ 5 ................$2,000 May ......................... 7 ................$2,300 June ........................ 8 ................$2,700 July ......................... 2 ................$1,200

Using the high-low method, what is the cost equation