Lets the points:
A = (X1, Y1) = (0,2)
B = (X2,Y2) = (3,0)
note that X1 = 0, X2 = 3 and Y1 = 2 , Y2 = 0.
then the slope of the line that passes through A and B is by definition:
[tex]m\text{ = }\frac{Y2\text{ - Y1}}{X2\text{ - X1}}\text{ = }\frac{0-\text{ 2}}{3-0}\text{ = }\frac{-2}{3}[/tex]