It is impossible to know the exact equation of the graph without actually seeing the graph itself or atleast having more information. Therefore, using the information provided and assuming that the graph crosses both the x and y-axis at point (0, 0) then the equation would be the following
y = 1/3x
This equation shows that the graph has a slope of 1/3 and crosses the x and y axis at point (0, 0) like mentioned above and shows in the picture attached below.