1 - The right and the left side must weight the same, and the weight of both the right and the left side would be 24, therefore each side must weight 12.
2 - Since the heart = 2, and 2*heart + square = 12 , then square = 8
3 - Now, since square = 8 and square + moon = 12, then moon=4
To summarize
heart = 2, square = 8 , moon = 4.