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 Shipped	Total Shipping 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

2. Using the high-low method, estimate the cost formula for shipping expense.