Can the volume be non positive? I have a question please 
I have asked my teacher if the area could be negative she said :no never
But what about the volume ? E.x 
If $y=x+2$ and $y=-x-2$,$x=0$ about x-axis 
The volume become zero!And about y-axis it becomes in negative ! Why ?
Thanks all

Yes, volumes can be 0, but volumes can never be negative. The volume of a square is 0, for instance. You might want to look into measure theory and lebesgue measures.
