Answer :

y + -x is the same as y - x.  12/13 +(-1/13) is the same as 12/13 - 1/13. 
The question is now 12/13 -1/13.  

12 - 1 = 11.  The answer is 11/13.  


12/13 + (-1/13)
= 12/13 - 1/13
= 11/13

note the "+" and "-" turn it into a "-" which means you have to subtract.

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