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

Substitute  for m, 4 for x₁, r for y₁, r for x₂ and 2 for y₂ in slope form.

Apply cross products property: If then .

Distribute terms using distributive property: .

                (Multiply: )

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

  (Add 20 to each side)

           (Apply additive inverse property: )

                   (Apply additive identityproperty: )

                     (Subtract: )

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

        (Add 4r to each side)

               (Apply additive inverse property: )

                    (Apply additive identityproperty: )

                              (Add: )

                   (Multiply each side by negative one)

                                   (Product of two same signs is positive)                                

The value of .