Automatically create and update equation that represents the line (polynomial) of data set

Hi everyone, I'm new here so I hope I'm writing in the right Forum...

I have a data set, with an X and Y axis, that will be changing quite often. I know I can graph the data set and then display equation, but since the data will be changing often I would like it to be done automatically (I don't need the graph just the equation). The equation that best fits the data is a polynomial one (in all cases).

I found out that "Linest" function can do this, but when I tried using it it gave me a polynomial equation with 4 parts (ax^3+bx^2+cx+d) and the results didn't match the data nor this equation was like the one displayed by the graph (ax^2+bx+c) even though both were based on the same data.

An excel document has been attached.

I would appreciate any kind of help on this!

Thanks!