You can see that triangle YXZ and QPR are similar because they have 2 angles in common. This means that they have a scale factor between them. The scale factor between triangle YXZ and QPR happens to be 1/3, as you can see, side QP is 1/3 times of corresponding side YX. To find PR, you just need to check how long its corresponding side is. XZ is 27 units long, and 1/3 of 27 is 9 units.
