Remark
3 is a common factor. If you pull 3 out using the distributive property, you get 3(9 + 5)
You could do this with much more complex questions like 625 + 730
For the question you were given, You could factor this into their primes.
15: 3 * 5
27: 3 * 3 * 3
The highest common factor is 3
So that is what comes out. The rest is inside the brackets.
3(5 + 9) shows what is left over.
For the example I used
625: 5 * 5 * 5 * 5
740: 5 * 2 * 2 * 37 There is only 5 that is common to both of these.
5(125 + 148)