You can only hit boxes on the bottom row.
If a box is destroyed, the boxes above it will fall down.
If the numbers on adjacent boxes form a sequence starting with one, all of the boxes in that sequence are destroyed.
Which boxes do you need to hit to destroy them all with just three cannonballs?
Touch a cannon to fire it at the corresponding box.
Look for a box to destroy that will make the numbers line up.
The second shot will destroy the box with the 9 on it.
There are four boxes with 1 on them, and there is just one of those that you don't need to use in a sequence.
Find it and destroy it.
First, aim at the 1 on the left.
Then destroy the box that will allow you to make a sequence all the way from 1 to 9.
Use your second shot to destroy the 3 on the right.
Then destroy the box that will give you a sequence from 1 to 6.
The answer is as follows.
1. Shot to destroy the 1 on the left.
2. Shot to destroy the 3 on the right.
3. Shot to destroy the 3.