Answer
$\frac{F}{9}$.
Work Step by Step
The speed of a wave on a string, as shown in equation 15.14, depends on the tension and linear mass density.
$$v=\sqrt{\frac{F}{\mu}}$$
For the pulse to take 3 times as long to travel the length of the wire means that the speed has been reduced by a factor of 3. This happens when the tension F is reduced by a factor of 9.
