How do I offset something reletive to an entitys pan?

I want to put a sword on my players back for example, but the origin is on the handle of the sword, how can i move it right a bit in other words, how can i change its y pos relitive to the players y depending on the players pan..?

I want to offset the sword to the right by say 10, here is my current code:
vec_for_vertex(my.X,you,134); // get position of weapon
vec_set(my.pan,you.pan); // set pan to owner's pan
my.pan -= 90; // adjust weapon so player can see it better
my.tilt += 45; // adjust weapon so player can see it better