Average Bmi Females In Germany How Do I Find The Function That Describes My Data/plotted Graph?

How do I find the function that describes my data/plotted graph? - average bmi females in germany

Then he gave me a set of data: shows the body mass index of the U.S. average for women 2-20. How do I find a function (without using the regression tool) into my computer, that part of the graph is generated by plotting the data? And what does it mean when you made the "state variables and parameters?

I am very confused, please help: (