1. Stated problem: Solve for $a$ in the equation $a + 128 = 400$.\n\n2. To isolate $a$, subtract 128 from both sides: $$a + 128 - 128 = 400 - 128$$\n\n3. Simplify both sides: $$a = 272$$\n\n4. Conclusion: The value of $a$ that satisfies the equation is $272$.\n