Can someone point me in the right direction please.

I have the following
2 worksheets - named "results" & "Data"
Results sheet has 2 comboboxes, one sits on cell C5 in column "motorsize"
the other sits on D5 in column "motorname"
Combo 1 listfill range is from results sheet A20:A30 (linked cell C5)
Combo 2 listfill range is from results sheet D12:D14 (linked cell D17)
Data sheet has 3 columns,
A B C
1 size weg cemp
2 0.75 100 90

If I select 0.75 from the drop down list on combo 1, I then need to choose
the name from combo 2, the value of which is summed up elsewhere, how do I
get the information from data sheet so I can SUM the value in another cell in
reults sheet?
I hope that makes sense.