On earth time period,
\(T =2\pi\sqrt{\frac{l}{g}}\)
i.e., 3.5 = \(2\pi\sqrt{\frac{l}{9.8}}\)
Moon g = 1.7ms-1
T' = \(\sqrt{\frac{l}{1.7}}\)
Dividing \(\frac{T'}{3.5}= \frac{2\pi\sqrt{\frac{l}{1.7}}}{2\pi\sqrt{\frac{l}{9.8}}}\)
or T' = \(\sqrt\frac{9.8}{1.7}\times3.5\)= 8.4 s.