So we are given two points that the line crosses, the origin and (9, -3), we can calculate the slope m of the line with these data, dividing the y segment by the x segment:
m = (-3 - 0)/(9 - 0) = -3/9
m = -1/3
then we can use the point slope line equation to find the line equation, lets use the point origin (0,0) to do so:
y - y1 = m(x - x1), where x1, y1 are the coordinates of a point that the line crosses:
y - 0 = (-1/3)(x - 0)
y = (-1/3)x
so this is the equation of the line, slope -1/3 and y intercept 0
