Press Enter or Return to apply the new width.
Position the pointer inside the Free Transform bounding box, and then Shift-drag the
selected layer group down until the transformation values next to the pointer indicate a
vertical move of 1200 px (the Y value in the options bar should say about 1680 px when
the top right square of the reference point locator is selected). Press Enter or Return to
commit and exit the transformation.
Select the Left Column layer group in the Layers panel, and then choose Edit > Free
Transform.
In the options bar, do the following:
Make sure the Toggle Reference Point checkbox is selected, and then select the top left
square of the reference point location option.
Make sure the Maintain Aspect Ratio (link) button is selected, and enter 672px for
Width.