We have looked fairly extensively at waves on a string without anything on them, however there is a portion in my notes about an infinite string with a mass in the middle. Essentially the setup is that x=0 we have a mass. Here the waveform on each side must be equal as the string is continuous, that I get. However then we look at force considerations and we seem to place a negative sign on the B term and I have no idea where that comes from. Does anybody see why that would be in the attached picture?