For a linear or quadratic, inverting y=f(x) is simple algebra. Cubic or
quartic polynomials are not as simple, but
http://mathforum.org/dr.math/faq/faq...equations.html
should get you started.

There is no general algebraic solution to higher order polynomials, and
no requirement that there be a closed form inverse of a general function.

Given the functional relationship, you can numerically invert at a given
point using Solver.

If your "best fit" line is the smoothed line connecting the points
instead of a least squares trendline, then Excel appears to use cubic
splines for smoothing.

Jerry

Sara Beaver wrote:

Is there a way to use the formula and the line of best fit to plot a point on
the line using the y-value of an unknown in order to find the precise
location of the x-value?