I don't understand the following math problem.?

(Let's say "O" is theta) ume that the differential equation of a simple pendulum of length L is LO'' + gO = 0 where g = GM/R^2. .... A grandfather clock loses 10 minutes a day when the length of the pendulum is 30 in. With what length pendulum wil the clock keep perfect time? The answer is 29.59, but I don't know how to get to it, or to approach the problem. Help would be appreciated.