Sum of two consecutive numbers

The sum of two consecutive numbers is -21.  What are the two numbers? 

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x = first consecutive number
x + 1 = second consecutive number

x + x + 1 = -21   {sum of the two numbers is -21}
2x + 1 = -21   {combined like terms}
2x = -22   {subtracted 1 from each side}
x = -11   {divided each side by 2}
x + 1 = -10   {substituted -11, in for x, into x + 1}

-11 and -10 are the two consecutive numbers

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