Help > Authoring > Equations > Editing Equations > Making Multi-Row Equations
  
Making Multi-Row Equations
New equations start with a single row.
To add a row above or below the current row, choose the appropriate option from the Commands menu in the Equation Editor window.
To insert a new row below the current row, press the ENTER key. This moves the text to the right of the marker (if any) to the new row, splitting the row into two rows.
To join consecutive equation lines choose Commands > Join Row Below.