I have having trouble yet again!

x sub one + x sub 2 + x sub 3 = 15 These are all nonnegative integers and

x sub one is greater or = to 0 and less than or equal to 6

x sub 2 and x sub 3 are both greater than or equal to 0

Should I reduce k by 6 in the formula C(9 + 3-1,3-1)???? Please advise.