Answer
$\lambda_n=\frac{4l}{2n-1}$
Work Step by Step
Here, we have one node where the string can't move and one antinode where the string can freely move. The possible combinations of wavelength from length include:
$\lambda_1=4l$, $\lambda_3=\frac{4l}{3}$, and $\lambda_5=\frac{4l}{5}$
$\lambda_n=\frac{4l}{2n-1}$