P1 := x-y-z=7; P2 := x-2*y+z=1; n1 := vector([1,-1,-1]); n2 := vector([1,-2,1]); theta := angle(n1,n2); evalf(convert(theta,degrees),3); L := solve({P1,P2},{x,y,z}); v := crossprod(n1,n2); a := subs(z=0,L); a := vector([13,6,0]); Line := a + t*v; evalm(Line);