finding the serial number for A3

I bought a hard copy of A3 for my imac. I'm presuming it is okay to use the same disk to load A3 onto the macbook I have just bought. To do this requires a serial number. I can only see one number (7 digits and a back slash) on the original A3 box. However, this number doesn't seem to work. I have a vague feeling that the serial number may have been on a peel-off sticker that could have been lost. I sent a request to Apple for help but have not received a reply. It is important that I have A3 on my laptop. Help would be appreciated. jp.