(a) Each point on the y-axis is of the form (0,y,0). Such a point is on the plane P if
3(0)+21y+0=4⟺y=8
So the intersection of P with the y-axis is the single point (0,8,0).
(b) Each point on the z-axis is of the form (0,0,z). Such a point is on the plane P if
3(0)+21(0)+z=4⟺z=4
So the intersection of P with the z-axis is the single point (0,0,4).
(c) The intersection of the plane P with the yz-plane is a line.
We have shown in parts (a) and (b) that the points (0,8,0) and (0,0,4)
are on that line. Here is a sketch of the part of that line that is in the first
octant.
