Create new file.
Display the Rulers
View > Rulers
Drag the horizontal & vertical guide lines to get the center.
View > Guides > Lock Guides
Type the text. Select it with Arrow Tool.
Insert > Convert to Symbol
Name the symbol.
Symbol makes the work easy. You will find the cross mark at the center of the text.
Keep the Arrow Tool on the cross mark and drag the text to the center of the stage. Align exactly at the center.
Insert key frame at 10 and 20.

Click Scale icon on the standard tool bar (marked with green)

Select 1 st frame. Compress the text with the scale command.
Select 20 th frame. Again compress the text.
Create motion tween between 1 to 10 and 10 to 20.

Create second layer. Add blank key frame at 20. Drag the symbol at the center.
Modify > Transform > Flip Horizontal

Insert key frame at 30 and 40.
Select 20 th frame. Compress the text with the scale command.
Select 40 th frame. Again compress the text.
Create motion tween between 20 to 30 and 30 to 40.

Save the file