Finding stresses at a point

So I'm studying for an upcoming Mechanics of Materials exam. Gotta ace it! Anyways, I'm having trouble with these types of problems. Finding state of stress on a point in 3D. This picture comes from: http://www.academia.edu/6204133/Chapter_09

Which has worn shown, but not explanations. I have two questions to start, but hopefully more will surface and get answered . . .

1) I always have trouble finding Q and I don't know why. Does anyone have a solid equation or "explanation for dummies"? It's just the centroid-to-center-of-area distance, right?

2) Which equation to use and what it corresponds to. I think what would really help is just, what does each equation mean in terms of stress. For example, I know Normal Stress is σ = F/A, and I know it acts parallel to the plane it's on (so σx points in the X direction).

I think using this problem as an example could work. Also, feel free to move this if it's in the wrong place.

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