Get W|A to help derive new wave equation with anchors?

The wave equation assumes two points and a line between them.

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del ^2psi=1/(v^2)(partial^2psi)/(partialt^2)

I’d like to attempt a new wave equation taking into account additional anchors for the two points and their relationship. A simple example would be a taught bow (like bow and arrow) with string attached at both ends.

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 \_________/

How would I tell W|A the extra bow information?