Question

How do you find the sum of 12/13 +(-1/13)

Answer 1

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.  

Answer 2

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

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