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:
\(3(\ )\)
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.
\(27 \div 3 = 9\)
\(12 \div 3 = 4\)
So now put these back into the brackets and you're done:
\(3(9+4)\)
Try to factorise \(18+12\) now to make sure you understand.
