Answer
$T=596N$
Work Step by Step
We know that;
$v=\sqrt\frac{T}{\mu}$
This can be rearranged as;
$T=\mu v^2$
As $\mu=\frac{M}{L}$,
Hence $T=\frac{M}{L}v^2$
We plug in the known values to obtain:
$T=\frac{800\times 10^{-6}}{0.22}(405)^2=596N$