1. The problem is to calculate the sum of 309_264 and 678. 2. First, note that the underscore in 309_264 is likely a formatting separator and does not affect the number's value, so the number is 309264. 3. The formula for addition is simply adding the two numbers: $$309264 + 678$$ 4. Perform the addition step-by-step: $$309264 + 678 = 309942$$ 5. Therefore, the final answer is 309942.