Firstly look for the highest number which goes into both of them:
1 goes into both of them
3 goes into both of them and is the highest so we use this one.
Next write a 3 outside a set of brackets:
Then, the point of these brackets is that you times everything inside them by 3 so in order to get the numbers inside, you divide both by 3.
So now put these back into the brackets and you're done:
Try to factorise
now to make sure you understand.