A common ratio is just that number that you multiply or divide to each number that gets you the next in a geometric sequence. In this sequence, we know the numbers are getting bigger and we're multiplying, so we want to know what you're multiplying by the first number to get to the second. We can call this unknown number x. SO if we start with the first number and set up an equation, we get:
3x = 12
Divide both sides by 3
x = 4
Therefore, your common ratio is 4.
To check, you can multiply all the next numbers by 4 to see that it all checks out.
