Question 888639
Let's name the numbers x and y.
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x + y = 12, the sum of the two numbers
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x-2y = 9, where one number is substracted twice from the other.
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Now, we just use one of the rules for solving two equations with two variables.
I suggest this method:

x +  y = 12 -> x = 12-y
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x - 2y = 9 ->  12 - y - 2y = 9
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12 - 3y = 9
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-3y = -3
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y = 1
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x = 12-y = 11
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Answer, 11 and 1! You can verify yourself. :)