$\"Slope$ $\"\"$

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Slope formulae $\"\"$ .

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

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$\"\"$

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Apply Cross multiplication: $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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Subtract 8 from each side.

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$\"\"$

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$\"\"$

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Divide each side by negative 1.

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$\"\"$

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$\"\"$ .

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$\"\"$

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The value of r is 12.