Tricky IF formula

[twolfend]

I'm trying to make cells display blank provided a certain field has no value. If the field does have a value, I need the displayed value to be a sum formula. Can't figure it with a sumif formula though... Help appreciated.



ie:
if c20<1, then display nothing, if true display sum(d7:d9)

So frustrating I've spent an hour trying it!

Thanks