$\"\"$

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The equation is $\"\"$ .

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$\"\"$ (Apply Distributive property: $\"\"$ )

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According to addition property of equality; if $\"\"$ , then $\"\"$ .

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$\"\"$ (Add 21 to each side)

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

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$\"\"$ (Add: $\"\"$ )

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$\"\"$

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$\"\"$ (Divide each side by 7)

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$\"\"$ (Cancel common terms)

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

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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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$\"\"$

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

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$\"\"$ (Multiply: $\"\"$ )

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Compare the values, the equation is true.

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$\"\"$

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The value of $\"\"$ .