The plane $2x - y + 3z + 5 = 0$ is rotated through $90^\circ$ about its line of intersection with the plane $x + y + z = 1$. The equation of the plane in the new position is:
Parabola is defined as the locus of points equidistant from a fixed point (called focus) and a fixed-line (called directrix).
=> MP2 = PS2
=> MP2 = PS2
So, (b + y)2 = (y - b)2 + x2