This sort of problem is quite common.
This kind of exercise is very popular.
In the above sentences, the expressions kind of and sort of are used. I'm not sure whether those are the subjects of the above verb or determiners. When translated in the first language, those are regarded as determiners or adjective, but I haven't identified the category of determiner includes those in google.(also including a variety of) Also, I think those might be different according to some situations like the example. So, my question is what those serve as in the above sentences.