You have not really stated what exactly you are struggling with, whether you just want someone to check your answers or what?
This is a moments problem so the first thing you need to do is to draw the free body force diagram. Part a) should become clearer when you do this. One assumption that you make here is that the ruler has a uniform density, so that the centre of mass is in the middle of the ruler and for this problem, it tells you where to place the weight (force) of the ruler.