Answer: 6 unit
Explanation:
Here, two triangle PQR and JXY are given,
In which QS and XZ are the medians of triangles PQR and JXY respectively,
Also,
![\triangle PQR \sim \triangle JXY](https://img.qammunity.org/2020/formulas/mathematics/college/j0hx7zu4nwi4x027s9x5o1jidvkgut5641.png)
Since, the corresponding sides and corresponding median of similar triangles are in same ratio,
![\implies (PQ)/(JX) = (QS)/(XZ)](https://img.qammunity.org/2020/formulas/mathematics/college/7676nul15hfj11jlfh7hejzv3l0rsn3jwg.png)
Here, PQ=9 unit , XZ=4 unit, QS=XJ unit,
![\implies (9)/(QS) = (QS)/(4)](https://img.qammunity.org/2020/formulas/mathematics/college/bap21xgkjqntbib76e9okxtboy057ovjwl.png)
![\implies (QS)/(9) = (4)/(QS)](https://img.qammunity.org/2020/formulas/mathematics/college/lnqbb4zos2i7izz5kvdq15i85qxh3lv30y.png)
![\implies QS* QS = 9* 4](https://img.qammunity.org/2020/formulas/mathematics/college/xeuuij71bxi2nub81l4kblvco46eql5h63.png)
![\implies QS^2 = 36](https://img.qammunity.org/2020/formulas/mathematics/college/bmxvjqq0n6oow5egp8roymmth6m1317s80.png)
Note : Since, it is the measurement of length, this is why we did not take √36 = - 6.