

Right-click Strikethrough in the Font group on the Home tab in the Ribbon.

To add Strikethrough to the Quick Access Toolbar: If the Quick Access Toolbar is above the Ribbon, click the down arrow to the right of the Quick Access Toolbar and select Show Below the Ribbon from the drop-down menu.

It's usually easier to work with the Quick Access Toolbar if you display it below the Ribbon. You can also add Strikethrough to the Quick Access Toolbar and then access it using Alt.
