Hi I would like to know the exact way to simplify an algebraic expression emphasizing on when and why I change a portion of it from a positive to a negative or vice versa?

Ok thank you. The most important part of my question that I need help with though us understanding when and why you changed - (7+6 - 2x) into - 7 - 6 - 2x. How did you know to change the + into a - between 7 and 6 but NOT to change - 2x into + 2x?