Khlee, when I copied your example data and used that formula in cell E6, my result was 19200. What result did you get?
When I changed the value of A1 from 24 to 36, the result for E6 was 28800. I'm not sure why it didn't work properly for you.
To explain how the ROW()-5 portion of the formula works, it basically instructs Excel to calculate January as 1 (since January appears in row 6, it takes the current row and subtracts 5). When you copy the formula down to December, it will increment each row by adding one.
So my formula equates to:
[TABLE="class: grid, width: 500"]
<TBODY>[TR]
[TD]Cell
[/TD]
[TD]Calculations
[/TD]
[TD]Result
[/TD]
[/TR]
[TR]
[TD]E6
[/TD]
[TD]=9600*2*1
[/TD]
[TD]19200
[/TD]
[/TR]
[TR]
[TD]E7
[/TD]
[TD]=9600*2*2
[/TD]
[TD]38400
[/TD]
[/TR]
[TR]
[TD]E8
[/TD]
[TD]=9600*2*3
[/TD]
[TD]57600
[/TD]
[/TR]
[TR]
[TD]E9
[/TD]
[TD]=9600*2*4
[/TD]
[TD]76800
[/TD]
[/TR]
</TBODY>[/TABLE]