Put the verb in brackets into the past perfect.
(reject) You knew that Mr Harisson our offer. Why didn't you tell me?
We build the past perfect using the auxiliary verb "have" in the past tense and the past participle of the main verb:
had + past participle
Regular verbs form the past participle with the -ed ending — exactly like in the past simple.
play — played — played want — wanted — wanted finish — finished — finished