Can I quit after working for few days?

Hi, I was offered a job this tue by company A and will start work next week. A has done a retrenchment exercise this year and not very stable. I was offered by my ex company today which i have worked 8-9 years. My ex companly is more stable. I thought of quitting company A and join my ex company which i have worked for 8-9 years. But I really do not know how to tell company A, I think it is not ethical too. Please advise