Does it matter? plz help!?

i juz had an interview in an optical shop n i was hired to work on the 1st of october and i was so happy! but after that, i juz saw a banner stated nid to hire a tutor (my favourite job)! they asked me to come for an interview on this coming monday! but the matter is.....if i get the tutor job, how should i tell the optical company? i feel embar n i dont know wat to say! i oldy promise him to come to work for him but now i oledy found my favourite job!