The constant of proportionality is 2/3x. This is because the equation states that the equation/line will travel constantly without speeding or slowing down.
y=ax+b
->a= positive (the line is going up)
->to find a, we move one unit horizontally and look over how many units you meet the graph (1 to right, 4 up) so the a= 4
-> to find b, we look for the y-coordinate of the point where the graph and the y-axis cross. they cross in (0,1) so b is 1
when we take this all together we get y=4x+1
