I have curve and someone made an equation. OK,
the questiones now are:
1-how can I think to make an equation?
2-what is the level of validity of this equation?
thank u very much for helping

Please explain a little better what you are trying to do.

For example, do you have a table of values of x and y, and are you trying to interpolate to find an intermediate value? Or are you trying to use regression or some other technique to forecast the y value corresponding to the next x value?

And are you looking to perform statistical tests, or are you looking for a non-mathematical answer?