1. **Stating the problem:** Factorise the given expression (not specified, so assuming a general approach). 2. **Formula and rules:** To factorise an expression, we look for common factors, use special products formulas like difference of squares, perfect square trinomials, or factor by grouping. 3. **Example:** Suppose the expression is $x^2 - 9$. 4. **Apply difference of squares formula:** $$a^2 - b^2 = (a - b)(a + b)$$ where $a = x$ and $b = 3$. 5. **Factorisation:** $$x^2 - 9 = (x - 3)(x + 3)$$ 6. **Explanation:** We rewrite the expression as a product of two binomials whose product gives the original expression. If you provide a specific expression, I can factorise it step-by-step.