Sometimes the** common factor** that you need to **factorise** an expression is not a number, but a letter.

Look for the highest power of each letter that will go into **every term** of the expression.

This will be your** common factor** to put outside the bracket.

Have a look at this example to see this in action.

**e.g. Factorise 3a ^{3} + 2ab.**

There is no number which is a common factor of **2** and **3**.

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**.

This means we need to put **a** outside of the bracket:

**a(__ _ __)**

But what goes inside?

We need to divide both terms by **a** to calculate this.

3a^{3} ÷ a = 3a^{2}

2ab ÷ a = 2b

So our final answer is:** a(3a² + 2b)**

In this activity, we will factorise expressions into a single bracket using letters or numbers as common factors.