Answer:
Well, there can be two answers. One of them can be 9, since they planted 9 MORE trees on top of the 12 already there. (21-12=9) The other answer can be 33 if you count all the trees they planted. (12+21=33)
Explanation:
The Smith's planted 9 trees
21-12=9
Because the Smith's ending amount is 21, and their beginning amount is 12 you subtract 21-12=9
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