This is essentially for a Calc II class, can anyone help? You'd be my hero, and I'll give you a kiss or something:

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Let P be a point not on the plane that passes through the points Q, R, and S. Show that the distance d from P to the plane is:

d = absolute value( (a X b) . c )
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absolute value( a X b )

where a = vector QR, b = vector QS, and c = vector QP.

("." signifies dot product, "X" signifies cross product)