The proportional relationship k is calculated as the ratio between the dependant variable and the independent variable.
If the relation is linear, it can be calculated using any point of the line.
For any linear relationship it can be calculated as:
If the point (0,0) k can be calculated directly by k=y/x.
Graphically, k is the slope of the line.
In this case, if we take the point x=2 shirts and y=16 dollars, the proportional relationship constant k is:
Then, as it has be done with several points, we know that we have a proportional relationship, as the quotient y/x is constant through all the points.
If there is no proportionality, the value of y/x is not constant.