Where Does the Last Equality in Equation 6.79 Come From?

[ehrenfest]

Homework Statement

Sorry this page is not part of the free preview but I would recommend that everyone buy their own Zwiebach (or download it illegally if ethics don't stop you).

Anyway, on page 109, can someone help me figure out where the last equality in equation 6.79 comes from?

Thanks.

Homework Equations

The Attempt at a Solution

 

[StatusX]

It comes from equation 6.78 (that might sound like a guess, but I really did check the book).