Well, to get the power series representation, you first have to take a derivative. Then, at the end, you have to take an integral, and when you take an indefinite integral, you have to add "+C" to it to represent any constant that the derivative could have knocked out. Then to solve for "C," you set x equal to 0, so you get..

ln(5-0) = 0 + C

ln(5) = C

I imagine that's what they're getting it from