Okay, so you have your two planes

1: x+2y-z=4

2: 2x+y+z=8

I assume you've figured out by now that the normal to plane 1 is going to be in the direction (1, 2, -1) and the normal to plane 2 is going to be in the direction (2, 1, 1). I guess you then used the scalar (dot) product between them to work out what the angle was.

The next bit: Unit vector in the line of intersection.

Well, the unit bit just means it has a total length of 1 so let's worry about that later. First of all, work out what direction it's in.

How do we do this? Think about your two planes. If you're having trouble visualising it, hold up some sheets of A4 to help you. Each plane has its own normal coming out of it at 90 degrees.The line of intersection is in BOTH planes, right? (fairly obviously, since it has to be in both for it to be the line of intersection)

If a line is in a plane, then it's perpendicular to the normal of that plane. If it's in both planes,then it must be perpendicular to. Again, that has to be true, by definition pretty much. Think about that for a minute and convince yourself that it's right.bothnormals

If you want to generate a vector perpendicular to two other vectors, how do you do it? That's right. The vector (or "cross") product.

So it'll be in direction (1, 2, -1) CROSS (2, 1, 1).

Once you know the direction, you just have to divide by the total magnitude to make sure it has length 1.

Hope this helps.