The constant of proportionality is the value of y/x. Picking a random value on the graph, such as (2, 10), we can substitute this into the formula for k = y/x. 10/2 = 5. This satisfies one portion of the answer. Now, if we take the value of x from the same point, (2, 10), which is 2, and multiply it by 5, as illustrated in y = 5x, we get 10 for y. This suits the equation as mentioned and we can check other points with the same process to ensure that this answer is correct.
