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You only use De Broglie when you are finding the wavelength of something with a mass( wavelength = h/p) . p=mv, where m is mass and v is velocity. So, they would probably give you the two of the three variables and then you would solve for the third one.
DeBroglie determines the wavelength/wavelike properties of things with mass. So you don't use it for light, just things with mass (and velocity since you need momentum which is mass x velocity). You would probably use it in a problem to find wavelength, mass, or velocity (h is a constant)
Light does have a momentum. The photons however do not have a mass. So I suppose if the problem gave you a value for the momentum (p), you could use the de Broglie wavelength as normal. Otherwise, this equation doesn't work for light.
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