M2 - Centre of Mass help

Normally you'd have a table like:

____________ Small shape 1 ________ Small shape 2 ________ Small shape 3 ________ Whole shape
Mass ___________ m1___________________ m2 _____________ m3 _________________ M
Centre of mass _(x1, y1) _______________ (x2, y2) __________(x3, y3) ______________ (xbar, ybar)

If there's a hole, then you're subtracting off its mass to get the whole shape.

How would you do part b?

Im struggling with part b of this question and with questions in this sort of area. What is the main method for answering such questions because my textbook doesn't cover it very well.
What are the best ways of answering centre of mass questions, especially like this one and how do you answer this one?
Thanks

Try a search - this has already been asked and explained. Sorry - I don't have the thread handy.

EDIT: It's here.

Draw a line from the point of suspension (S) and add on the mass, then take moments.

Thanks but i've already read through it and really don't understand, could you help? How would you attempt a question like this in general?

How would you attempt a question like this in general?

The centre of mass of a composite body is just a weighted average of the CoM of each component. In this question, there's symmetry about the x axis, so we already know the y co-ordinate is 0. You therefore take the weighted average of the x coordinates of the centre of mass of the two objects, one of which has a negative 'mass' (or area).

I'd do the same for the final part, calculating the x coordinate from the given angle.

some questions friction acts vertically up from the wall and in others it acts down, how do you know?

You can make an educated guess, but it doesn't matter if you're wrong. All that will happen is that you'll get a negative value for your friction force, indicating that it's in the opposite direction to what you'd assumed.