Theory
Factorising is the reverse of expanding brackets. Instead of removing brackets, you are putting brackets back in.
A factor is something that multiplies to make a term. For example, 6 is a factor of 6x and 12.
Look for the highest common factor shared by every term. The factor must divide into all terms, not just one of them.
Put the common factor outside the bracket. Then divide each original term by that factor to fill the bracket.
Check your factorisation by expanding it again. If it expands back to the original expression, it is correct.
If the question says factorise fully, take out the highest common factor, not just any common factor.
Key ruleab + ac = a(b + c)