$\"\"$

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The slope m of a non vertical line through any two points is given by,

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$\"\"$

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$\"\"$ (Substitute 4 for m, $\"\"$ for x₁, $\"\"$ for y₁, r for x₂ and $\"\"$ for y₂) $\"\"$

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$\"\"$ (Product of two same signs is positive)

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$\"\"$ (Apply cross products property: If $\"\"$ then $\"\"$ )

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

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

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

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$\"\"$ (Add 2 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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$\"\"$ (Divide each side by 4)

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$\"\"$ (Cancel common terms) $\"\"$

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The value of $\"\"$ .