the equation is
y = mx + b
Where m is the slope from (0,9) to (3,0), and b is the y coordinate where the line touches the y axis.
The slope from (0,9) to (3,0) is -3. That's because when x increased by 3, y decreased by 9. -9/3 = -3
The line touches the y axis at y=9 because that is when x=0. We get this from the point (0,9).
The equation of the line is: y=-3x+9
