Chapter 9 - Basic Algebra - 9.2 Adding and Subtracting Signed Numbers - 9.2 Exercises - Page 642: 71

(a) $-6-6=-12$ (b) $-9-5=-14$

(a) When changing the operation from subtraction to addition, you must also change the sign of the subtrahend. The problem should have been re-written as $-6+(-6)$, not as $-6+6$. (b) The sign should not have been dropped from the first number in the equation. The problem should have been re-written as $-9+(-5)$, not as $9+(-5)$.