Hi Arturo,

Not a lot of information to work with there, but try this.
Add a series to your chart that references two cells, the first cell has
your
x value = 90 and the second has your y value = (a calculation that returns
the same value as your trendline when x=90).
Then you can format the series error bars to show a line from the series
point back to the axis line.

HTH
Martin