Answer
(a) subtract
(b) negative number
(c) additive inverse; negative number
Work Step by Step
(a) In the phrase $6-9$ the "-" sign indicates a subtraction operation because it is written between two numbers.
(b) In the phrase $(-9)$ the "-" sign indicates a negative number. There is no minuend to indicate a subtraction operation.
(c) In the phrase $-(-2)$ the first "-" sign indicates the additive inverse of the number that is in parentheses. The second "-" sign indicates a negative number. There is no minuend to indicate a subtraction operation.