The prefix с-, со- has the sense of together or joined; it is similar to the English prefix com-, con-. The root лег(ж)-, лаг(ж)-, лог(ж)- has the sense of lay, down, or put. So, сложный means put together from parts. The English combining form -plex means "having parts or units": duplex, triplex, cineplex, multiplex, etc. It comes from Latin with the meaning of fold, twine, or braid.
сложить - to fold, lay down, add, pile up слагаемый - component, summand, term
налагать/наложить - to impose, put on, lay on; налог - tax полагать - to think, deem. suppose отложить - to postpone, defer, put off принадлежать - to belong (to), be possessed (by)
Also, DL is marking wrong "this" (for "это") in this sentence. Is this just an oversight that they need to correct, or does "это" really not mean "this" in this sentence?
The Hints for "это" change with different sentences. Can "это" change its meaning that much based on context? I've seen both "this" and that" in the Hints but then sometimes one or the other is marked wrong. I'm confused.
OK, this is a complicated one and I'm not a native speaker but I'll try and help.
There are two forms of это - the neutral pronoun and the definition/identifying pronoun. The neutral version is only used for neutral nouns (ending with о/е if you remember) and is the same as this. это здание = this/that building. It usually means this but depending on the context can mean that as well.
The identifying form of это is used when trying to explain what something is and can be used with any gender noun. это - карандаш = this/that is a pencil. It again can be written as that, but it depends on the context.
There are notes and comments explaining this stuff much better but I'm not sure where they are. If I find them, I'll update for you.