so if we were to integrate both sides-- well if you integrate this side, you're taking the integral of a derivative, you just get back to what you took the derivative originally of, so this just becomes f of x times g of x, and then we have to integrate the right hand side, well that just becomes-- and we're doing the indefinite integral, kind of the antiderivative, but we can use this tool when we do definite integrals as well.