Do you have Excel?. It does a find job of regressions.
I get a cubic that fits well. It has an R^2=1
I guess one way to do it would be to plot the data, or create a mental image of the graph.
Then, as it appears to be a cubic, you can sub in co-ordinates into the general cubic equation, then solve the simultaneous equations to get the unknowns.
You could then test this equation using the other x and y values.