All right, I get it. It explains why I couldn't follow some steps with physics when they rewrote functions. It seemed complicated but it is quite simple.

The remainder has to be zero because the definition of divisionable is that there has to be no reamainder.

So it turns out that I had two problems with this question:

I hadn't learned how to execute a polynomial division and I didn't know the definition of divisionable. This definition is, for me, counterintuitive. I assumed that every fraction wich is a rational number (domain of

) would be defined as divisionable.