If you will look at the graph , it is a straight line passing through Origin(0,0) , and if you produce the ray or line from both ends it will lie in first and third Quadrant.
So, equation will be of the form
y=k x, where k will be any positive real number.
To find equation of line we need two distinct points.
The two points are , (0,0) and (1,2).
Equation of line passing through two points will be

⇒Constant of Proportionality will be either 2, or
.