Who solved that problem?

By whom is that problem solved?

By whom will that problem be solved?

By whom can that problem be solved?

By whom was that problem solved?

Active Voice: Who

Passive Voice: By whom

This is a sentence of Past Indefinite Tense.

Was and Were are the helping verbs of Past Indefinite Tense.

Active: Who solved that problem?

Passive: By whom was that problem solved?