In the equation y = 5x, the constant of proportionality is the number that relates y to x. Here, the constant is 5.
This means that for every unit increase in x, y increases by 5 units. The relationship described by this equation is linear, indicating that y is directly proportional to x. Thus, you can think of the constant of proportionality as the slope of the line when this equation is graphed, where the slope represents the rate of change between the two variables.