r1:-x+2*y+z=0; r2:x+y+2*z=0;

linsolve([r1,r2],[x,y,z]);

load(draw)$

draw3d(surface_hide=true,xaxis=true,yaxis=true,zaxis=true,color=red,implicit(r1,x,-10,10,y,-10,10,z,-10,10),implicit(r2,x,-10,10,y,-10,10,z,-10,10));

r3:-x+2*y+z=7; r4:x+y+2*z=12;

linsolve([r3,r4],[x,y,z]);

draw3d(surface_hide=true,xaxis=true,yaxis=true,zaxis=true,color=blue,implicit(r3,x,-10,10,y,-10,10,z,-10,10),implicit(r4,x,-10,10,y,-10,10,z,-10,10));

draw3d(surface_hide=true,xaxis=true,yaxis=true,zaxis=true,color=red,implicit(r1,x,-10,10,y,-10,10,z,-10,10),implicit(r2,x,-10,10,y,-10,10,z,-10,10),color=blue,implicit(r3,x,-10,10,y,-10,10,z,-10,10),implicit(r4,x,-10,10,y,-10,10,z,-10,10));