I have never used the coordinates other than y up, z toward, and x at 90, as shown in SolidWorks. When I did the practice exam I placed the components as oriented on the page, not noticing the negative marks on x and z, so when I checked the mass properties it said my center of mass was as shown on answer D, when in fact it should have been answer C. (see attachment) Can anyone point me in the direction to figure this out. I don't have a problem with the parts, just orienting them to -x,y,-z. I don't understand which way is up, down, positive, or negative. It makes no sense to me.