Translation:Please take a look at this agreement.
Good question, I did a small search and found the following:
In essence they mean the same thing but the law states that 合同 is a 协议 entered into by parties of equal bargaining powers, so there's a subtle, if legally non-existent, difference there. However, in a non-legal context, 协议 can mean both a contract, which is legally binding, and an agreement, which may or may not be. In light of that, while it seems duolingo means "agreement", both should be acceptable.
'qì yuē' is a more formal term for "contract" or "agreement", one could even call it business and legal jargon. As in English, it covers the many areas where an agreement or a contract is used between parties through speech, actions, or in writing.
Source: 百度百科, sort of the Chinese version of Wikipedia. https://baike.baidu.com/item/契约/2874688
For a more comprehensive and rather too detailed for duolingo understanding, check out https://wiki.mbalib.com/wiki/契约. It basically says that 契约 is equivalent to the concept of a "contract" in English, which in normal usage may or may not be legally binding, in part or in whole.