For this exercise it is important to remember the following:
The constant of proportionality, which is represented as "k", describe the constant ratio of two quantities that are proportional (the independent and dependent variables).
In this case we know that the independent quantity represents the total members of the band and the dependent quantity represents the woodwind members of the band.
Since the band consists of 3 woodwinds for every 7 members of the band, we can conclude that the constant of proportionality for this relationship is: