If it is 5pm the east of south america, what till will it be in western south america?

Time is always "earlier" in the east and "later" in the west - think of it like this, the Sun rises in the East, so it will rise earlier in the East!

There are three time zones in South America, so the difference will be 2 hours: so it will be two hours later, that is, it will be 7 PM!