What is the frequency of a vibrating steel cable in its third harmonic?

[MasterPnut]

Homework Statement

A steel cable is vibrating in its third harmonic. The Frequency is 150Hz
1. What is the length of the cable?

2. What is the wavelength?

Homework Equations

The Attempt at a Solution

I am really stuck

 

[Simon Bridge]

Thats OK - we can help with that.
Where do you get stuck?

Do you know what is meant by the "third harmonic"?
Do you know about "standing waves"?