@tomiki
As leocg said, you are probably on 64bit, else you would be stuck on opera 45. To check if you are on 32 or 64, open a terminal and run this
uname -m
If it says "x86_64" you are on a 64bit mint 17.3, if it says i686 you are on 32. There are may other ways to check that, but this is the one that I remember now.