Would you please solve it real quick, that will be helpful and appreciated. Thank you. I will give you a good feedback too.

In the figure, a string, tied to a sinusoidal oscillator at P and running over a support at Q, is stretched by a block of mass m = 0.58 kg (this is like lab #2). What is the frequency of the oscillator associated with the fourth harmonic on the string (as shown below) if the separation is L = 0.9 m and the linear density of the string is u = 1.1 g/m? Oscillator