I have an idea of the concept this problem is trying to ask but I can't quite figure out how to solve it.

Given that:

integral from 0 to 3 f(x) dx= integral from 0 to 1 f(x) dx + integral from 2 to 1 f(x)dx + integral from 2 to 3 f(x)dx

Find integral from 1 to 2 f(x)dx

I don't really know how to think of it. I know an integral equals area under the curve but can't really see what else i'm missing.