Remember that any MacBook whose model name is "Late 2008" or older cannot run OS 10.10 due to hardware limitations. They are limited to a max OS of 10.7.5
That assumes you have a Macbook and not a MacBook Pro; too often those names get used interchangeably and they are NOT the same computer.
If you are not sure of your model, try entering the serial number in this Apple page:
https://selfsolve.apple.com/agreementWarrantyDynamic.do
It will return a model name such as:
If that does not work (may not for older computers) you can go to this site:
http://www.everymac.com/ultimate-mac-lookup/
On that site, you only need enter the LAST three characters of the serial number, the way I recommend as it is not an Apple-oowned site.
Let us know what you find.