Final answer:
The constant of proportionality in the equation is 2.
Step-by-step explanation:
In the equation Start Fraction x over y End Fraction = Start Fraction 2 over 9 End Fraction, the constant of proportionality is the ratio of the numerator and denominator of the fraction on the right side of the equation. In this case, the constant of proportionality is 2. This means that for every 2 units increase in x, there will be a corresponding increase of 9 units in y.