The y-intercept (where the line passes the y-axis) is positive 9.
The line is decreasing so the slope is negative.
The slope of a line can be found by the equation :
y2 - y1 / x2 - x1
Substitute the coordinates.
0 - 9 / 3 - 0
-9 / 3
-3
The slope is -3.
So, your equation in slope - intercept form is :
y = -3x + 9
