1. **State the problem:** Simplify the expression $a (6x + 15) - - 18$. 2. **Rewrite the expression:** The double minus sign before 18 means plus, so the expression becomes: $$a(6x + 15) + 18$$ 3. **Distribute $a$ over the terms inside the parentheses:** $$a \times 6x + a \times 15 + 18 = 6ax + 15a + 18$$ 4. **Final simplified expression:** $$6ax + 15a + 18$$ This is the simplified form of the given expression.