The acceleration due to gravity at height 2R from the surface of the Earth will be,
g′=(R+h)2gR2⇒g′=(R+2R)2gR2⇒g′=9g
Length of the second's pendulum at the surface of the Earth is 1m.
Since, time period of a pendulum is given by,
T=2πgl⇒TT′=ll′×g′g⇒1=ll′×g′g⇒l′=9l=91m