Here's how you can stack the words "monthly rental pmnts" in one cell in Excel:

1. Select the cell where you want to stack the words.
2. Click on the "Wrap Text" button in the "Alignment" section of the "Home" tab. This will allow the text to wrap within the cell.
3. Type in the first word "monthly" and press "Alt + Enter" on your keyboard. This will create a line break within the cell.
4. Type in the second word "rental" and press "Alt + Enter" again.
5. Finally, type in the third word "pmnts" and press "Enter" to complete the cell.

The words should now be stacked on top of each other within the cell. You can adjust the height of the cell to fit the text by dragging the bottom border of the cell downwards.