That is a 3, 4, 5 right triangle.
Since 5 × 3 = 15 everything is multiplied by 3
3 × 3 = 9 ( the smaller leg )
4 × 3 = 12 ( the longer leg )
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Answer : 9
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I hope that helps you out!!
Any more questions, please feel free to ask me and I will gladly help you out!!
~Zoey