Question 636935
Add 12 to -9 to get 12+(-9) = 3


So -9 and 3 are the same number on the clock (ie going backwards 9 hours is the same as going forwards 3 hours...you land on 3 each time)


So you must find a number x such that 


3 + x = 0


Solve for x:


3 + x = 0


3 + x  - 3 = 0 - 3


x = -3


So -3 is the inverse of 3


Add 12 to -3 to get 12+(-3) = 9


This means that 9 is the inverse of -9

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Useful shortcut:


The additive inverse of 'x' is -x


Ex: x = 6 ---> additive inverse: -x = -6


Ex: x = 7 ---> additive inverse: -x = -7


etc etc