Answer
$40\,Hz$
Work Step by Step
There are two full wavelengths in the string and the string length is given to be $2.0\,m$.
$\implies \text{Wavelength } \lambda=1.0\,m$
$\text{Frequency } f=\frac{v}{\lambda}$ where $v$ is the wave speed.
$\implies f=\frac{40\,m/s}{1.0\,m}=40\,Hz$
