1. **State the problem:** Factorise the expression $3x + 12$. 2. **Recall the factoring rule:** To factorise an expression, find the greatest common factor (GCF) of all terms and factor it out. 3. **Find the GCF:** The terms are $3x$ and $12$. The GCF of $3$ and $12$ is $3$. 4. **Factor out the GCF:** Write the expression as $3(x + 4)$ because $3x = 3 \times x$ and $12 = 3 \times 4$. 5. **Final answer:** The factorised form of $3x + 12$ is $$3(x + 4)$$. This means the original expression is the product of $3$ and the binomial $(x + 4)$.