原句是：(假设)买了汽车支付...
The nobleman sold his car to repay Farmer Fleming.
英语不像中文,如果用英语写疑问句必须使用一个助动词（原句里有助动词或者情态动词时除外）.
已有助动词：
--He will go there alone.
疑问：Will he go there alone?
已有情态动词：
--He should go there.
疑问：Should he go there?
你给的句子没有助动也没有情态动词,就要在疑问句里加上一个助动词.又由于原句使用了过去时,所以要用did.这个助动词did要放在主语前面.What did the nobleman do...