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

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The expression is $\"\"$ .

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According to addition property of equality: if a = b, then a + c = b + c.

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$\"\"$ (Add 31 to each side)

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$\"\"$ (Additive inverse property: $\"\"$ )

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$\"\"$ (Add: 8 + 31 = 39) $\"\"$

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

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$\"\"$ (Subtract 12x from each side)

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$\"\"$ (Commutative property: a + b = b + a)

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$\"\"$ (Additive inverse property: $\"\"$ )

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

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

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$\"\"$ (Divide each side by 2)

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$\"\"$ (Cancel common terms)

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$\"\"$ (Divide: $\"\"$ ) $\"\"$

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The value of $\"\"$ .