This question was put on hold recently (and answered in the comments).
I do not question the decision but would appreciate further guidance about the particular reason so I can adjust future questions accordingly. Also I am not requesting the question be re-opened.
The question is not homework (I cannot tell whether it is "homework-like"). It is a simple calculation-oriented question so maybe that qualifies. It is certainly about physics and requests a definite answer.
The comments were helpful and the point seemed to be that I must come to grips with prerequisites before dealing with a topic that depends on them. Point taken. But that doesn't really apply to the question as much as it does to me. The question was well edited by Qmechanic (who actually went to the book and added the original equation numbers) and it seemed to me that a straightforward answer might be helpful to others who jumped into Feynman's book even if they did have the background.
Parenthetically, I did not show my own work because I didn't think it would help anyone to do so. My hope was that by seeing the answer I could make some necessary inferences about the topic (variational calculus and Lagrangians) and proceed further. Of course Feynman gives many hints in his text to allow this to some extent anyway but an authoritative answer can lend one confidence.
Also parenthetically, this doesn't mean I didn't do work before posting the question. I am not working in an academic setting so I don't have professors or fellow students to consult.
Any further guidance about the specific deficiencies of the question would be appreciated. The question was answered generously in the comments so it's more or less moot but I would like to have a clear sense of what is not acceptable so I can steer clear of it.