There are 500 bacteria at the beginning and they double every 2 hours. So, if you want to find the number of the bacteria after 2 hours, you must multiply 500 by 2; after 4 hours - multiply 500 by 4; after 6 hours - multiply 500 by 8; etc.
You can write it like this:

where x is the number of the bacteria after t hours

There will be 2,048,000 bacteria after 24 hours.