Translation:Two sad Finns are looking for a black cat.
So apparently the exception to the rule is when the numeral is preceded by words like nämä, "these", or nuo, "those", in which case what is being referenced is definite and the verb is plural. Without those words, the numeral does not definitely refer to anyone or anything in particular. In this case, we only know something about two sad Finns, not which sad Finns they are in particular.
Words in the sentence change depending on where they are located and what context is used, it is not possible to explain this properly without using terms like partitive and object. If you are serious about learning Finnish, you NEED to know about partitive. You can check some information here, for example https://uusikielemme.fi/finnish-grammar/grammatical-cases/the-partitive-case-partitiivi
This is such a big question! Here it is used because of the numeral, and both the noun and the adjective have to be in partitive. And then partitive is used after “etsii”. When it comes to forming partitive and when it is used, I’d recommend checking out a grammar book or other online resource, there is too much to describe here. There is a good thread with questions that are frequently asked here, that you might want to take a look at https://forum.duolingo.com/comment/40624627
From what I've been seeing it isn't quite that simple as the definite article presence, and plural verb use is more of an exception. Simple case: "Kaksi suomalaista" is a complex noun phrase and a singular nominative, hence, kaksi etsii. If it was "nämä kaksi suomalaista", then the subject with "these" is plural. There is a good discussion here https://forum.duolingo.com/comment/40624627/Unofficial-grammar-tips-for-all-Finnish-learners-FAQ-list-Updated-on-August-12th Also, Annika has posted this link in that same thread http://www.kielitoimistonohjepankki.fi/ohje/318 . From what I understand, the plural verb use has to be preceded by an indication that the subject with the number is something very particular, like here the examples all have something like "nämä", "jonka", "Big Brotherin", "Baltian", etc., and there seems to be certain word order too... I'm not a linguist and not a native speaker, so keep that in mind. Just sharing what I found useful on that topic.