User function to solve Polynomal equation

[Not So Smart]

I have made several scatter charts and inserted a polymonal (6) trend line. These charts have several hills and valleys. Knowing my value along the (x) axis (in this case a date) I can visually (with a ruler) find the approximate value on the (y) axis. I would like to write a macro to find the (y) value when given the (x) value.

Does anyone know of a userfunction that has been written to solve a polynomial shaped line given the (x) value.

Thanks

David