Martin, consider the following:
1: Is this y=1.083333333x^4-7.3333333x^3+16.416667x^2-6.16667x+4 the correct
equation? This would be a polynomial equation with the largest exponent of
four. If not, then the equation has a degree of one and is a linear equation.
2: Also, for information sake, there is a previous entry on this chat site
concerning solving quadratic equations by Dave Braden. He referred to a
particular textbook that is accessable on the web; quoting him, "You can get
this straight off of the web by searching for "Numerical Recipes in C", then
look for Chapter 5. section 6 (Quadratic and Cubic Equations), pp 183 ff.
<quote If either a or c (or both) are small, then one of the roots will
involve the subtraction of b from a very nearly equal quantity (the
discriminant); you will get that root very inaccurately. The correct way
to compute the roots is
q = -(b + sgn(b)*sqrt(b^2-4a*c))*0.5

Then the two roots are
x1 = q/a and x2 = c/q
<end quote

let us know how you manage,
CJ


"MartinW" wrote:

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