In a 3D architectural model, several planes represent different walls and roofs. Consider three planes defined by the equations:

Plane P1: x + y + z = 6
Plane P2: 2x – y + 3z = 4
Plane P3: -x + 4y – z = 2

Your task is divided into two parts:

(a) Identify which two of these planes intersect along a line that forms a non-right angle with each other. Provide a brief explanation for your choice.

(b) For the pair you identified in part (a), compute the acute angle between the two intersecting planes using the formula for the angle between planes: \(\cos\theta = \frac{|\mathbf{n}_1 \cdot \mathbf{n}_2|}{\|\mathbf{n}_1\|\,\|\mathbf{n}_2\|}\), where \(\mathbf{n}_1\) and \(\mathbf{n}_2\) are the normal vectors of the respective planes.