Hi,

1. I've got as the result of the cross product: [-1, -3, -7]

2. Therefore the equation of the common line would be: [x,y,z] = [1,2,-1] + t[-1, -3, -7]

3. The angle between 2 planes is equal to the angle between the normal vectors of the planes. Use the the definition of the dot product to calculate the angle.

If the angle between the planes is called then I've got: