a pendulum clockis lag by 2 min during a whole 24 hours.what correction should be made in its length so that it keep the right time?

The clock runs slow by a factor

24.0333/24 = 1.00139 That its the ratio of the actual period to the desired peiod

The period is proportional to the square root of the length. Therefore the length should be shortened by a factor
(1/1.00139)^2= 0.9986

That is another way of saying the length of the pendulum should be decreased by 0.14%