In a function, each domain element can have at most one range element. So we'll have to remove one of the links from 3. Either one can be removed, since it is OK for two domain elements to map to the same range element (as is the case for 3->b and 4->b).
This is all very theoretical, but here is a real life example.
Take f(x) = x²
You probably know that both f(2) and f(-2) is 4.
There 2 and -2 are in the domain, 4 is in the range.
