1. **Problem:** Find the factor of $(a + 5)(a - 9) - 15$ from the given options. 2. **Formula and rules:** Use distributive property to expand and simplify expressions. 3. **Work:** $$(a + 5)(a - 9) - 15 = a^2 - 9a + 5a - 45 - 15 = a^2 - 4a - 60$$ 4. Factorize $a^2 - 4a - 60$: $$a^2 - 4a - 60 = (a - 10)(a + 6)$$ 5. The factors are $(a - 10)$ and $(a + 6)$. Among the options, $a - 10$ matches. **Final answer:** $a - 10$ (Option খ)