1. The problem statement, all variables and given/known data Here are the screenshots of the textbook... Code (Text): http://i37.tinypic.com/nce7md.jpg http://i36.tinypic.com/8zpwew.jpg What I don't understand is the parameter (x - vt) I am confused by the picture. This is why I can't even ask a precise question, and rather I can only ask a very general question, and I must apologize... Question: Can someone explain to me what the picture shows, and how do I see (x - vt)? I see vt = d from mechanics. But the pictures are very confusing... Sorry. and thank you!