Sometimes the common factor that you need to find to be able to factorise an expression is not a number, but a letter.
Look for the highest power of each letter that will go into each of the terms of the expression. This will be your common factor to put outside the bracket.
Have a look at this example:
Factorise 3a3 + 2ab.
The common letter in both terms is a. The highest power of a that goes into both terms is 1, so the common factor is a. You need to put a outside of the bracket.
Put 3a2 + 2b inside of the bracket. This is because when you multiply a and 3a2 you get 3a3 and when you multiply a and 2b you get 2ab, which is the original expression.
The final answer is a(3a² + 2b).