Where was the person served.
Disclaimer: This answer does not constitute legal advice. I am admitted in the States of New York, New Jersey and Massachusetts only and make no attempt to opine on matters of law that are not relevant to those three States. This advice is based on general principles of law that may or may not relate to your specific situation. Facts and laws change and these possible changes will affect the advice provided here. Consult an attorney in your locale before you act on any of this advice. You should not rely on this advice alone and nothing in these communications creates an attorney client relationship.
This affidavit of service form is not very good. As an affidavit, it would need to be notarized but no notary signature block is provided. If it were considered as an unsworn declaration under RCW 9A.72.085 it would have to recite that it was true under penalty of perjury. But as to how to fill out the form, both previous answers are correct. On line 3 you would put the address where the defendant was served. At the bottom you would put the server's address.