
Adam V. answered 10/03/16
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Senior Software Engineer, over 16 years work experience!
Let n = The normal vector for the plane = <9, -1, -1>
The general equation of a plane is:
n * <x-x0, y-y0, z-z0> = 0
<9, -1, -1> * <x-7, y+8, z+4> = 0
9(x-7) -1(y+8) -1(z+4) = 0
9x - 63 - y - 8 - z - 4 = 0
9x - y - z = 63 + 8 + 4
9x - y - z = 75