1. **State the problem:** Simplify the algebraic expression $54x^3 + 27x^3 - 10x - 4a$. 2. **Combine like terms:** The terms $54x^3$ and $27x^3$ are like terms because they both contain $x^3$. 3. **Add the coefficients of like terms:** $$54x^3 + 27x^3 = (54 + 27)x^3 = 81x^3$$ 4. **Rewrite the expression with combined terms:** $$81x^3 - 10x - 4a$$ 5. **Final simplified expression:** $$81x^3 - 10x - 4a$$ This is the simplest form since the terms $-10x$ and $-4a$ are unlike terms and cannot be combined further.