couldn't you calculate that using just B2 and C2?
As per my comment in Chris' answer - updated version
This should avoid the problem inherent in Chris' suggestion in that you get the wrong value when the first value <=0 is 0 itself.
$B:$B<0 returns an "array" of
TRUE/FALSE values, the first
TRUE obviously co-inciding with the first column B value <0, MATCH then finds the position of that first instance and INDEX then gets the corresponding value from column A.
The second INDEX is only there to avoid "array entry" - it works without that, i.e.
.......but that version would need to be "array entered" - i.e. confirmed with
It's a little more inefficient than the previous suggestions, using the whole column (and that doesn't work in Excel 2003 or earlier - in those versions you need to use a specific range).
Note that MATCH with "match type" of -1 as per Chris' suggestion needs to have descending values in column B - this formula works however column B is ordered.