$\"\"$ $\"\"$

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The word “subtracted” indicates the symbol “ $\"\"$ ”.

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The word “is” indicates the symbol “ $\"\"$ ”

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Let $\"\"$ represents number. $\"\"$

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Rewrite the sentence into equation,

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$\"\"$

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The equation is $\"\"$

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According to subtraction property of equality: if $\"\"$ , then $\"\"$ .

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$\"\"$ (Subtract 21 from each side)

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$\"\"$ (Apply commutative property of addition: $\"\"$ )

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$\"\"$

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$\"\"$ (Apply additive inverse property: $\"\"$ )

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$\"\"$ (Apply additive identity property: $\"\"$ )

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$\"\"$ (Subtract: $\"\"$ )

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$\"\"$ (Multiply each side by negative one)

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$\"\"$ (Product of two same signs is positive)

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$\"\"$

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Check:

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To check the solution, substitute $\"\"$ in original equation.

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$\"\"$ (Substitute $\"\"$ )

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$\"\"$ (Subtract: $\"\"$ )

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Compare the values, the equation is true. The solution is $\"\"$ .

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$\"\"$

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The value of $\"\"$ .