Furnace A	Furnace B	Furnace C
Initial investment	$450,000	$400,000	$300,000
Annual Revenues*	$80,000	$80,000	$80,000
Annual cost*	$10,000	$8,000	$13,000
Salvage value	$50,000	$40,000	$35,000
life of asset	15 years	15 years	15 years

I am working on problem 6-7. I am having issues calculating the IRR method. I don't know where I am going wrong. The problem states:

Fiesta Foundry is considering a new furnace that will allow them to be more productive. Three alternative furnaces are under consideration. Perform an incremental analysis of these alternatives using the IRR method for each increment of cash flows. The MARR is 12% per year.

*annual revenue and cost figures are increases over the "do nothing" alternative