your complaint / I / happened / have investigated / and finally

your complaint / I / happened / have investigated / and finally discovered / what
【正确答案】：I have investigated your complaint and finally discovered what happened.
【题目解析】：本句句意为我已经调查了你的投诉，并最终发现发生了什么事。在本句中，what要引导宾语从句，所以根据句意及句子的基本结构，即主谓宾，答案如上。