what do you have to do to solve this
Try substituting the plane wave solution into the wave equation.
And what rule do you use to do that
You use, like, (d/dx)e^(f(x))=(d/dx)(f(x))*e^(f(x)). The ordinary rules of differentiation. Just tell us what you get. The exponentials should cancel on both sides leaving you with an algebraic relation between omega and k.
is there like a video that teach you the rules of ordinary rules of differentiation ?
I don't know that there is a video, but try any elementary calculus text. Pay particular attention to the 'chain rule'.
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