How to unclog sink/tub pipes?

[Justin Case]

A tenant just called and said all her pipes are clogged and drain slow. I know to take the U shaped pipe off the sinks and clean them.
I did this before she moved in.

What do I do if it drains slow after I clean these pipes again?

How do I clear the tub pipe that is not accessible? Only the tub drain its self is.
I was told before do never use Drain-no

Any tips for me when I go there tomorrow and try to fix these drain issues?

Thanks.