And these guys are all perpendicular to each other. Why? Well, because these lines, the Z and the X axis, the X and the Y axis, and the Z and the Y axis are all perpendicular to each other. So all these three guys PX, PY, and PZ must be perpendicular. Now, our range for our Magnetic quantum numbers can be derived using our L. Our range begins at minus L and goes to plus L. So our PX begins at negative L.
And since this guy represents L equals one, that means our PX must be L minus one or minus one. So our ML is minus one.