Geometry

In quadrilateral ABCD, $$\overline{BC} \perp \overline{AB}$$, $$\overline{DA} \perp \overline{AB}$$. The coordinates of B are (4, 2), and the coordinates of C are (2, 4). Point P (1, -1) lies on $$\overline{AD}$$. The equation of $$\overline{AD}$$ is: y = __________.