One step of backward propagation on a computation graph yields derivative of final output variable

To explain the above sentence, I found following computation from {moderator edit: link and image removed}

But I still can`t understand how that backward propagation thing works to compute the derivative :confused:

Iโ€™m deleting the link from your post, since it is distributing the course materials without permission. Thatโ€™s not allowed by the course honor code.