VECTOR angle!

[poohead]

hey guys

how do i find the acute angle btw planes 2x+y-2z=5 3x-6y-2z=7??

I keep getting 70.89 degrees
but according to the book the answer is 79.0

:/

help please?

[xZero]

isn't this more like uni maths? but anyways the normal to the plane 1 is (2,1,-2) and the normal to the plane 2 is (3,-6,-2). The angle between the planes is the angle between the normals so use angle =cos^-1(dot product/the multiple of the two modulus) and you should get 79.02 degrees