What is the period of this pendulum?

[brad sue]

Hi,
I have this problem:

A simple pendulum of length L is suspended from the ceiling of an elevator. The elevator is accelerating upward with acceleration a.
What is the period of this pendulum?

From logic I know that T=2pi*sqrt(L/(g+a)), but when I try to find it .. I failed.
Please can I have some help to find the period ?

 

[mezarashi]

It appears to be the right thing to do. Check calculations?

 

[brad sue]

It appears to be the right thing to do. Check calculations?

I just to help me to set up the equation .
I think I can handle the rest.

B