Finding the differential equation for a oscillating system

[JustinLiang]

Homework Statement

The Attempt at a Solution

I don't think this is the correct answer because for some reason I have a constant mg term. Usually I get mgsinθ and from small angle approximations it becomes mgθ, but this time I am getting mgcosθ and from small angle approximations it becomes mg.

What am I doing wrong? Or is this correct?

 

[haruspex]

I don't see the distances in your expressions for the moments.

 

[theBEAST]

I don't see the distances in your expressions for the moments.

I just tried this question and did it with the distances, but I also don't understand why there is a constant term. That term is throwing me off.

 

[haruspex]

I just tried this question and did it with the distances, but I also don't understand why there is a constant term. That term is throwing me off.

Please post your working if you'd like further assistance.