The first thing to do is to work out the biggest number that goes into bother 24 and 3. As the only that goes into three- other than 1- is itself, one can assume this is the number we are looking for.
Next, divide bother numbers by 3.
24 / 3 = 8
3 / 3 = 1
You now take 3 out of the brackets, and leave 8 and 1. 24x / 3 = 8x.
3(8x + 1)
