How to get the regression curve equation

Hi! I'm working in a biology laboratory and often we have to measure concentrations of proteins. We calculate a calibration curve with standard concentrations of a know protein.
Then we write down the measure and the concentration in a way like this:
0 ug/uL - 0.342
5 ug/uL - 0.401
10 ug/uL - 0.450
15 ug/uL - 0.498

etc..

So we need a program that takes these measures, put it together in a graph, and make a curve (linear) that cross all those points, and I want the equation of this curve, so I can use it with my experimental data.

In excel it's easy, Just called "regression curve equation" , and in Open Office it's the same too.
But i'd like to find one in Numbers .. but I couldn't find until now this option

can u tell me if I can find somewhere or not?
thank you very much!
Luca