Point-slope form is
y - y1 = m(x - x1), where the (x1, y1) is one of the points.
First find the slope, which is change in y over change in x, or
[tex]m= \frac{x2-x1}{y2-y1} [/tex]
The example points
(x1, y1) and (x2, y2) are now
( 3, 21 ) and ( 6, 12 ).
[tex]m= \frac{12-3}{6-21}= \frac{9}{-3} = -3 [/tex]
Now plug the slope and one of the points into the formula
[tex]y - y1 = m(x - x1) \\
y - 12 = -3(x-6) \\ [/tex]
To write in slope-intercept form, y = mx + b, simply solve for y.
[tex]y - 12 = -3x + 2 \\
y = -3x + 30 [/tex]
