Solid Company manufactures and sells specialty items. The following representative direct labor- hours and production costs are provided for a four-month period:

Month	Hrs. Direct Labor	Production Costs
January	3,000	$45,000
February	4,000	52,500
March	7,000	81,000
April	4,000	45,000
Total	18,000	$223,500
a = fixed production costs per month
b = variable production costs per direct labor hour
n = number of months
X = direct labor-hours per month Y
equals total monthly production costs
Using the symbols above, indicate the cost estimation equation based on number of direct labor hours per month, and calculate total monthly production costs for May using the high-low method, assuming the direct labor hours for May are expected to be 4,500.