$\"\"$

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The equations are

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$\"\"$ Equation (1)

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$\"\"$ Equation (2)

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$\"\"$ Equation (3) $\"\"$

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Multiply the equation (1) by 3.

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$\"\"$ Equation (4)

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Add equation (4) and Equation (2).

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$\"\"$

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Multiply the equation (5) by 3.

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$\"\"$

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Multiply the equation (3) by 5.

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$\"\"$

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Subtract equation (7) from Equation (6).

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

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Divide each side by 4.

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$\"\"$

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$\"\"$

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Substitute $\"\"$ in equation (6).

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$\"\"$

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$\"\"$

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Subtract 9 from each side.

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$\"\"$

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$\"\"$

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$\"\"$

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Divide each side by negative 15.

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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Substitute $\"\"$ and $\"\"$ in equation (1).

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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Add 21 to each side.

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$\"\"$

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$\"\"$

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$\"\"$

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Divide each side by negative 6.

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$\"\"$

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$\"\"$

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$\"\"$

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The solution is $\"\"$ .