Let P1 be the plane 3x - y - 7z = 11 and P2 be the plane passing through the points (2, -1,0), (2,0, -1), and (5, 1, 1). If the foot of the perpendicular drawn from the point (7, 4, -1) on the line of intersection of the planes P1 and P2 is (α, β, γ), then α + β + γ is equal to _____.
