i guess the problem is asking for the length of the cable.
the system can be assimilated to a simple pendulum, whose period is given by:
[tex]T=2 \pi \sqrt{ \frac{L}{g} } [/tex]
where L is the length of the cable and g=9.81 m/s^2.
By re-arranging the formula and using T=3.8 s, we find the length of the cable:
[tex]L= \frac{T^2}{(2 \pi)^2} g = \frac{(3.8 s)^2}{(2 \pi)^2} (9.81 m/s^2)=3.6 m[/tex]
