You have a string with a mass of 13.7 g. You stretch the string with a force

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Question:

You have a string with a mass of 13.7 g. You stretch the string with a force of 9.99 N, giving it a length of 1.91 m. Then you vibrate the string transversely at precisely the frequency that corresponds to its fourth normal mode, that is, at its fourth harmonic. What is the wavelength of the standing wave you create in the string?

What is the frequency?