$\"\"$

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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 $\"\"$ )

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

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Apply cross products property: If $\"\"$ then $\"\"$ .

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$\"\"$

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Distribute terms using distributive property: $\"\"$ .

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$\"\"$

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$\"\"$

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Apply addition property of equality: If a = b then a + b = b + a.

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$\"\"$

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

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

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Apply addition property of equality: If a = b then $\"\"$ .

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$\"\"$

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

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The value of $\"\"$ .