1. **State the problem:** We are asked to find the sum of the numbers in problem 9: $+394 - 170 - 212 + 621 + 205$. 2. **Formula and rules:** To find the sum of a list of numbers, add all positive numbers and subtract all negative numbers. The order of addition does not affect the sum. 3. **Calculate step-by-step:** $$394 - 170 = 224$$ $$224 - 212 = 12$$ $$12 + 621 = 633$$ $$633 + 205 = 838$$ 4. **Final answer:** The sum is $838$. This method applies to all similar problems: add and subtract sequentially to find the total sum.