try1

Steps for making the trigger

1. first have the shapes inserted or pics.
2. then add an arrow like structure facing down which will be the trigger to subtract  boxes 
3.  After that press on animations to make for two of the boxes 
4. while doing the animation use the exit fly down option and press on the trigger on click then the down arrow 
5. do both boxes at the same time through adding on the second animation with previous