Answer
The tension in the cord is 4.8 N
Work Step by Step
We can find the speed $v$ of the wave as:
$v = \frac{L}{t}$
$v = \frac{8.7~m}{0.85~s}$
$v = 10.24~m/s$
We can find the tension $F_T$ in the cord as:
$v = \sqrt{\frac{F_T}{\mu}}$
$F_T = v^2~\mu$
$F_T = \frac{v^2~m}{L}$
$F_T = \frac{(10.24~m/s)^2(0.40~kg)}{8.7~m}$
$F_T = 4.8~N$
The tension in the cord is 4.8 N.