How do I figure out the value in the varying cash flow series to where it is positive?

0 -1000


1 -1000


2 -1000


3 -1000


4 500


5 1500


6 2500


7


8


9 ?








I need to find the value for year 9 that makes the net cash flow positive.I know that I need to use the NPV function and the What if Analysis in order to find it...but it keeps giving me crazy answers....HELP!!!