A picture is enlarged by keeping its width the same and increasing its length. The scatter plot shows the perimeter of the picture (y) for different lengths (x):

Since you have not included the plot, I'll just tell you how to find the function of the graph that you are looking for.
Function of the straight line has the general formula: y = mx + c
where: 
m is the slope of the line and c is the y-intercept

So first we will need to find the slope (m):
Taking two points from the graph (x1,y1) and (x2,y2), the slope can be calculated as follows:
m = (y2-y2) / (x2-x2)

Then, we will need to find the y intercept.
we have the equation y = mx + c and we have m calculated
Take any point that belongs to the line and substitute with the values of x,y and m and then calculate c.

Now you have both m and c calculated, just substitute with the values in the formula.