You can use Solver

Name a cell x
Name a cell y, enter formula:
=A20*x^4+B20*x^3+C20*x^2+D20*x+E20

Lauch Solver (ToolsSolver)
Target: y
Tick Value and enter 25
In Variable cells, enter : x
Click Solve

Et voilà!

HTH
--
AP

"MartinW" a écrit dans le message de news:
...
y = 1.083333333x4 - 7.3333333x3 + 16.416667x2 - 6.16667x + 4

Above is the polynomial equation of a trend line as shown on my chart,
I have extracted the numbers into separate cells using LINEST, so now
I have in cells A20 to E20 the numbers 1.083333333, -7.333333,
16.416667, -6.16667 and 4 respectively.
With a "y" value of 25 how do I go about solving for a value of "x".

I've been stubbornly struggling with this on and off for a couple of weeks
now and I'm not getting any closer to working it out and the websites
I've found explaining the procedure all seem to go over my head
by the time I get into the fourth or fifth paragraph. Any help would be
greatly appreciated.

Thank you
Martin