Final answer:
The constant of proportionality in the equation y = (5/9)x is (5/9), representing a direct relationship between y and x.
Step-by-step explanation:
The constant of proportionality in the equation y = \dfrac{5}{9}x is \dfrac{5}{9}. This constant is represented by the coefficient of x in the equation that signifies a direct proportionality between the variables y and x. In proportional relationships represented by linear equations of the form y = kx, where k is the proportionality constant, any change in x will result in a change in y that is k times as large. Consequently, if we plot this relationship on a graph, we will see a straight line that passes through the origin (0, 0).