Concept section: last example says normal vector is <0,0,-4> for z = -4, but it should be <0,0,1>

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Jason, you're absolutely right.  Referring to the concept video, last example, we can think of the plane z = -4 as 0x+0y+1z = -4.  Thus the normal vector would indeed be <0,0,1>.  Thanks for letting us know!