how to enter the equation generated by a trendline in a spreadsheet

I created a data set with the dates of swim meets and the time for the race for each meet. I am trying to use the function generated by the trendline to see if it can predict if my daughter will reach a certain time by a certain date. the best fit is the 6th degree polynomial equation. But when I enter the equation in a new cell to try and predict a time for a specific date I get a negative number. I am converting the date into a serial number before entering it in the equation.
The formula generated by the trendline is y=-7E-23x^6+2E-17x^5-2E-12x^4+8E-08x^3-0.0023x^2+35.939X-23187

How do I specifically enter this equation into excel

Thanks

Hello,

have a look at the LINEST() function. Study the help files. It's a bit much for a post and can take a while to get your head around.

cheers,