Fill in the blank number -350 + = -150

The answer is 200. I figured this out because if you have to add 2 positive numbers to get this: 350 + 200= 550 then with the negative numbers it would become this: -350 + 200 = -150.

I hope this helped! If it is too complicated just tell me. I can explain it easier.

3 7 is the answer I hope this helps