Given, Acceleration due to gravity on the surface of earth =9.8m/s2
Time period of the pendulum on the surface of the earth =3.5s
T=2π√lg
Where, l=length of the pendulum
l=T2g4π2
l=3.5×3.5×9.84×3.14×3.14
l=3.043 m
Given, Acceleration due to gravity on the surface of moon =1.7 m/s2
As length of the pendulum remains constant,l=3.043 m
Time period of the pendulum on the surface of the moon,T=2π√lg
T=2×3.14√3.0431.7
T=2×3.14×1.338
T=8.4 s.