$\"\"$

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The expression is $\"\"$ .

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$\"\"$ (Write the expression)

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

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

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

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

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$\"\"$ (Multiply each side by 4)

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$\"\"$ (Cancel common terms)

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

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The solution is 72. $\"\"$

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Check:

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Substitute $\"\"$ in the equation.

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$\"\"$ (Write the equation)

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

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

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

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The solution was checked. $\"\"$

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The solution is $\"\"$ .