'The equation of the parabola.
Parameters
x : str, optional
Label for the x-axis. Default value is \'x\'.
y : str, optional
Label for the y-axis. Default value is \'y\'.
Returns
equation : sympy expression
Examples
>>> from sympy import Parabola, Point, Line
>>> p1 = Parabola(Point(0, 0), Line(Point(5, 8), Point(7, 8)))
>>> p1.equation()
-x**2 - 16*y + 64
>>> p1.equation(\'f\')
-f**2 - 16*y + 64
>>> p1.equation(y=\'z\')
-x**2 - 16*z + 64'

def equation(self, x='x', y='y'):

    x = _symbol(x)
    y = _symbol(y)
    if (self.axis_of_symmetry.slope == 0):
        t1 = ((4 * self.p_parameter) * (x - self.vertex.x))
        t2 = ((y - self.vertex.y) ** 2)
    else:
        t1 = ((4 * self.p_parameter) * (y - self.vertex.y))
        t2 = ((x - self.vertex.x) ** 2)
    return (t1 - t2)
