@Carl Love  it works. Thank you a lot. JJ.

@Carl Love  Thank you very much. Using your suggestion I modified my L1 and L2.

L1:= {4*x + 3*y + z = 0, x + y - z - 15 = 0, z=t}:

L2:={12*x + 5*y + 7*z -13 = 0, 9*x + y -3*z - 5 = 0, z=t}:

p1:=solve(L1, {x, y,z});

p2:=solve(L2, {x,y,z});

But..what do I need to get a solution of the equations in parametric form?

solve(p1,p2, {x,y,z}, 'parametric');

(The solution in my toy example must be empty..but anyway I cannot use directly solve() ).

@Kitonum  Thank you very much for your help. Kind regards, JJ

@YasH  Thank you for your comment.Regards. JJ

about how to use maple.

ANyway, thank you.

Jeanj

Thank you for the help.

jj

@Markiyan Hirnyk 

thank you very much.

jj