There's a way without using Linest:
select the series in the graph and then select Chart - Add trendline.
If you want to use Linest nevertheless, proceed as follows.
Select two adjacent cells (beside each other) and type the formula
=LINEST(B2:B20;A2:A20)
(assuming your x values are in A2:A20 and your y values are in B2:B20).
Enter the formula with Ctrl-Shift-Enter and not by just pressing enter (this is a special formula called CSE formula or array formula).
Next, assuming the the LINEST results are in D1 and E1, put the following formula in C2:
=A2*$D$1+$E$1
and copy it down along the values in column B. Now add this series to your chart with Chart - Add data.
Marc