Since we have the point (0,0) we already know that the y-intercept=0. And since the line is just:
y=mx+b, where m=slope and b=y-intercept, and we know b=0 we have a line of the form:
y=mx, now we just find the slope which is (y2-y1)/(x2-x1)=-8/2=-4 so
y=-4x