Direction Numbers for Line of Intersection of 2 Planes:
plane P1: 2x - y + 3z = 4 and
plane P2: x + 3y -4z = 2
The line of intersection, L, lies in both planes, and hence is perpendicular to the normal vectors of both planes. The cross product, n1 ´ n2, is also perpendicular to n1 and n2, so it can be used as a reference vector in the direction of the line.
From the equations, we can determine the normal vectors:
n1 = 2i - j + 3k, and
n2 = i + 3j - 4k.
Now we form the cross product:

=-5i + 11j + 7k
Hence the direction numbers are -5, 11, 7.