The constant of proportionality is the slope.
The slope is the change in Y over the change in x. (Y2 -y1) / (x2 -x1)
From a table use two set of values (x,y) subtract two y values from each other and then subtract the two corresponding x values from each other. Depending on the size of the table it’s a Leah a good idea to use the first and last set of values from the table.
From a graph locate two points on the line that you can get an accurate x and y value and subtract the two y values and the two x values: (y2 -y1) / (x2-x1).
From a written description there should be a reference two two different points, using the two referenced point subtract them like in the directions given above.