I was hoping some could help shed light on the idea of a Point Of Action of a force.

I have a rectangular wall of 32m which is holding back water, the water is at a height of 29m.

Where is the point of action of the force?

The problem is illustrated below:-

I have been told that to find the point of action of the force for a rectangular wall I need to use the following equation:-

I need some help applying this because it does not make a lot a sense to me.

From research and help from the community about other questions, I feel that the point of action should be related to the centroid, but I am struggling to put all the pieces together.

Can anyone help?

Thank you.