What is the slope-intercept form of the function that contains the points (3, 1) and (2, 9)?

First let us find the slope of the line using the two points (3,1), (2,9) given.
x1 = 3, y1 = 1,  x2 = 2, y2 =9

m = (y2 - y1)/(x2 - x1). 
m = (9 - 1)/(2 - 3).
m = 8/-1 = - 8.
Therefore  slope, m = -8.

Using y = mx + c.  Slope = m,  m = -8.
y = -8x + c

Substituting any of  the point (3,1) into  the equation,  x = 3, y = 1.
1 = -8(3) + c
1 = -24 + c
1+24 = c
25 = c
c = 25  Substituting into y = mx + c,  m =-8,  c = 25
Therefore  y = -8x + 25.
y = -8x + 25.

That's it . Cheers.