'The equation of the line: ax + by + c.
Parameters
x : str, optional
The name to use for the x-axis, default value is \'x\'.
y : str, optional
The name to use for the y-axis, default value is \'y\'.
Returns
equation : sympy expression
See Also
LinearEntity.coefficients
Examples
>>> from sympy import Point, Line
>>> p1, p2 = Point(1, 0), Point(5, 3)
>>> l1 = Line(p1, p2)
>>> l1.equation()
-3*x + 4*y + 3'

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

    (x, y) = (_symbol(x), _symbol(y))
    (p1, p2) = self.points
    if (p1.x == p2.x):
        return (x - p1.x)
    elif (p1.y == p2.y):
        return (y - p1.y)
    (a, b, c) = self.coefficients
    return (((a * x) + (b * y)) + c)
