Khan is something like Duolingo. If you are scared of it and need to get comfortable, it's great. To learn seriously, you need something like a textbook with official reference points (defs, theorems, etc). If you seriously need that much practice for a given concept, then you're not learning it to begin with and need to focus on algebra fundamentals. That said, the examples in Khan often lack depth, so you won't necessarily see all the situations that illustrate the topic.