In order to determine the constant of proportionality of th given graph, you take into account tha the function that describes the line of the graph is:
y = kx
where y, is the value of the function, x is the independent variable and k is the constant of proportionality. You can calculate the value of k by solving the previous equation for k, just as follow:
k = y/x
where x and y can be whichever point of the graph. If you for example, use the point (2,30), where x=2 and y=30, and replace these values in the formula for k, you obtain:
k = 30/2 = 15
Hence, the constant of proportionality is k = 15