Answer :

Geekhawk
-x-1=x-21
Add 1 to each side
-x-1+1=x-21+1
-x=x-20
Minus x from each side
-x-x=x-x-20
-2x=-20
Divide -20 by -2x
If two negatives are divided, it becomes a positive number
-20÷-2x=10
x=10

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