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Finding Wavespeed

If you are given a history graph and a snapshot graph (or otherwise given the wavelength and period or frequency), you can use $v=\frac{\lambda }{T}$ or $v=\lambda f$ to find the wavespeed.

Alternatively, if you know that a crest travels a distance $d$ in a time $t$, then $v=\frac{d}{t}$ as usual.