Hope this helps.
If not, go to: https://www.mathportal.org/calculators/financial-calculators/simple-interest-calculator.php
You would have to add a positive 6 to the negative 6 to get zero. Lets say you have -2 in order to get it to zero or just any positive number, you have to add a positive of the same value or higher to be able to get it there. I hope you understand that.
X= -10
You may review the image Ive attached to see how I got this answer.