Final answer:
The constant of proportionality in the equation is 2/3.
Step-by-step explanation:
The constant of proportionality in the equation y = 2/3x is 2/3.
In a directly proportional relationship, the constant of proportionality represents the ratio of the corresponding values in the equation. In this case, for every unit increase in x, the value of y increases by 2/3 units.
For example, if x is 3, then y would be 2 units.