Activate your chart (double click) and do Insert > Statistics > Regression curves: (choose one).
Sorry, adding the equation of the regression to the chart is not possible with the current release of OOo. That feature is due for release with OOo 2.4 in about two weeks.
Meanwhile you can see the formula in the statusbar while the regression line is selected.
Please, edit this topic's initial post and add "[Solved]" to the subject line if your problem has been solved.
Ubuntu 18.04 with LibreOffice 6.0, latest OpenOffice and LibreOffice
acknak has a good description. If you also want pictures and some related info, see Show the equation of a curve in Calc. Also, you can download the release candidate before OOo 2.4.0 is released, but the RC may have some bugs.
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OOo 3.0.X on Fedora 9 + XP