Factorise 3a^2 - 9a

To factorise you need to identify 'factors', which are common features, between the different terms in an equation. In 3a^2 - 9a, the first term is 3a^2 and the second term is -9a. One common feature between these terms is that they both contain an 'a', and the other common feature is that they can both be divided by 3. Therefore, 3a is taken outside of the brackets. You then need to work what is left when each term is divided by 3a and this is put inside the brackets. So if you divide the first term by 3a, you are left with 'a' and if you divide the second term by 3a, you are left with -3. Therefore, the answer to the question is: 3a(a-3)