1. **Stating the problem:** Simplify the expression $3a - 2b + a - 5b$. 2. **Formula and rules:** To simplify algebraic expressions, combine like terms. Like terms have the same variable raised to the same power. 3. **Identify like terms:** Here, $3a$ and $a$ are like terms; $-2b$ and $-5b$ are like terms. 4. **Combine like terms:** $$3a + a = (3 + 1)a = 4a$$ $$-2b - 5b = (-2 - 5)b = -7b$$ 5. **Write the simplified expression:** $$4a - 7b$$ **Final answer:** $4a - 7b$