3,-6,9,-12… -20042007, - 2010 to find the sum of this column

3,-6,9,-12… -20042007, - 2010 to find the sum of this column

3,-6,9,-12… -2004,2007,-2010
=(3-6)+(9-12)+…… +(2007-2010)
=-3×2010÷6
=-1005