7. The reason because 15/5 equals three. 15 is the length of the bigger triangle, 5 the smaller. Because of this you get your scale factor of 3. Then use the scale factor to divide from 21 to get the length of the smaller triangle. The reason you are dividing is because you are going from a bigger triangle to a smaller triangle. 21/3=7, so 7 is the length of pr.
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