To save your formula you have written in libreoffice math press :kbd:`Control +S` or :menuselection:`File --> Save`. To open a file press :kbd:`Control +O` or :menuselection:`File --> Open`. To print your fomrula press :kbd:`Control +P` or :menuselection:`File --> Print`.
To paste text into libreoffice math press :kbd:`Control +V` or :menuselection:`Edit --> Paste`. To copy text from libreoffice math press :kbd:`Control+C` or :menuselection:`Edit --> Copy`. To cut text from libreoffice math press :kbd:`Control +X` or :menuselection:`Edit --> Cut`.
The left hand side has many symbols you can use for say set relations and other operators. To insert even more symbols :menuselection:`Tools --> Symbols`.