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General question about vectors. (point to plane)

Ok, this is more a general question...There's a picture if that helps for me to explain my question :).

Quote:

1/ Find the perpendicular distance of the point P(4, 5, 6) and the

plane x + 2y - 2z = 0.

This is the example in my text book. So i was reading through how the derive the formula

$\displaystyle PM = \frac{n . p}{| n | }$

Now what i don't understand with this...n is the same vector as PM...

So the modulus of n should be the distance of PM. But then if thats the case, the formula doesn't make sense because it would be saying

$\displaystyle |x| = \frac{x.y}{|x|}$

and we need to know |x| to work out |x|...therefore the formula is pointless...

I know this mustn't be the case and its just me not understanding.

Any help please :)?

Thanks