If your text already contains a formula, for example "12+24*2", LibreOffice can calculate, and then paste the result of the formula in your document, without using the \<emph\>Formula Bar\</emph\>.
Select the formula in the text. The formula can only contain numbers and operators and cannot contain spaces.
Choose Tools - Calculate, or press Command Ctrl+Plus Sign (+).
Place the cursor where you want to insert the result of the formula, and then choose Command Ctrl+V.
The selected formula is replaced by the result.