Do I just differentiate it twice? ...If so, to which respect to I differentiate it to?

Here's the problem:

You want to measure the mass m of an object, but you don’t have a scale. You therefore decide to attach the object to a string of mass ms and length L, as in the figure, and to generate standing waves on the string (pay attention to the orientation of the axes on the figure!!). The wave function that describes the standing wave is given by:

[tex]y(x,t)=\cos(\frac{2\pi}{\lambda}x+\phi)\cos(2\pi ft)[/tex]

where λ is the wavelength, f is the frequency, and φ is the phase.

a) Show that the wave function y(x,t) satisfies the

wave equation and from it derive and expression

for the speed of propagation of the wave in terms of

the given quantities.