Answer:
4L
Step-by-step explanation:
mass of pendulum = M, length of pendulum, L1 = L,
acceleration due to gravity = g, Amplitude = A
Use the formula for the time period for simple pendulum
..... (11)
Let the new length be L2 for which the time period is 2T.
..... (2)
Divide equation (2) by equation (1)

So, L2 = 4 L
Thus, the length of the pendulum is 4 L.