After inserting the object and the formula editor, the formula editor toolbar appears. First, click Fence Template in the lower left corner of the toolbar, select "{",then enter the equations one by one in the box flashing on the right side of the braces, and finally click outside the box.

If the symbol in the formula has an upper subscript, click "Subscript and Superscript Template" at the position of the symbol, select what you want, and type an upper subscript in the flashing box.