'The equation of the Plane.
Examples
>>> from sympy import Point3D, Plane
>>> a = Plane(Point3D(1, 1, 2), Point3D(2, 4, 7), Point3D(3, 5, 1))
>>> a.equation()
-23*x + 11*y - 2*z + 16
>>> a = Plane(Point3D(1, 4, 2), normal_vector=(6, 6, 6))
>>> a.equation()
6*x + 6*y + 6*z - 42'

def equation(self, x=None, y=None, z=None):

    (x, y, z) = [(i if i else Symbol(j, real=True)) for (i, j) in zip((x, y, z), 'xyz')]
    a = Point3D(x, y, z)
    b = self.p1.direction_ratio(a)
    c = self.normal_vector
    return sum(((i * j) for (i, j) in zip(b, c)))
