I will agree, you could be clearer about what exactly you want.

As 6stringjazzer notes, to obtain the z value at x=0, simply put 0 in for x in the trendline and compute y (which is just the constant term in the polynomial). For other values of x, simply put that value for x in and compute z.

From your original post, I first thought you were looking to give the equation z, then find x. For a 6th order polynomial, this is not possible algebraically, but one can easily use numerical algorithms to solve for x at a given z. Usually the easiest approach is to use Excel's built in Solver/Goal Seek utilities.

In all of these scenarios, the main "difficulty" to your problem is that you have used the chart trendline feature to perform the regression, so you need to transfer the equations coefficients into the spreadsheet. If this is a one time or rare task, then it is probably easiest to simply hand enter the coefficients into the spreadsheet. If this is something you will need to do frequently, I recommend that you become familiar with using the LINEST() function to perform polynomial regressions. The help file (https://support.office.com/en-us/art...a-fa7abf772b6d ) includes an example for a 3rd order polynomial. Extending that to a 6th order polynomial should be straightforward.