solve that (a=b=c)

Assuming for integers > -1 

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a-b-c=0 

a=b=c

2s=a+b+c  

2s-a-b=c 

a-b-2s-a-b 

-2b-2s=0 

-2(b+s)=0 

b+s=0  

Indeed for that to be true a+b+c/2 MUST be 0 and b is zero 

Their sum is 0 simply.