how do I use the forecast function in Excel?

I am trying to use the forecast function, but can not understand how to
decide what my "x" is or if I am trying to find the forecast of one set of
information, what are my "known Y" and "known X"? Any help on how you have
used this and perhaps a better description for each of the elements I need to
enter to use the formula.
thanks

See HELP for "FORECAST worksheet function"

known_x and known_y ranges combine to define two or more (x,y) pairs
that you want to model by a straight line. "x" is the x value of some
point on the line that want to know the y-value for.

If your known_x and known_y ranges contain more than two (x,y) pairs,
then the fitted line will pass "near" (in the least squares sense) all
the known values, but does not necessarily pass exactly through any of
the known values.

The assumptions under which the least squares fit gives is optimal
includes the assumption that measurement error for the x values is
negligible.

Jerry

Debs wrote:

> I am trying to use the forecast function, but can not understand how to
> decide what my "x" is or if I am trying to find the forecast of one set of
> information, what are my "known Y" and "known X"? Any help on how you have
> used this and perhaps a better description for each of the elements I need to
> enter to use the formula.
> thanks