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(21-s)/2=13
We move all terms to the left:
(21-s)/2-(13)=0
We add all the numbers together, and all the variables
(-1s+21)/2-13=0
We multiply all the terms by the denominator
(-1s+21)-13*2=0
We add all the numbers together, and all the variables
(-1s+21)-26=0
We get rid of parentheses
-1s+21-26=0
We add all the numbers together, and all the variables
-1s-5=0
We move all terms containing s to the left, all other terms to the right
-s=5
s=5/-1
s=-5
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