You could use Goal Seek to find X, given Y, or you could use the standard
quadratic equation solution (-B +/- SQRT(B^2-4*A*C))/(2*A)
--
David Biddulph

"DianeD" wrote in message
...
I have instrument calibration data that is best characterized as quadratic-
I
need to back-calculate for an adjusted point, using the basic formula y=
ax^2
+ bx + c.

I'm horrible at formulae in excel!