Alright, lets get started.
Please refer the diagram I have attached.
In the lower triangle, two angles are given as 60° and 90°.
The third angle could be find by subtracting these two angle from 180°. (Because the sum of angles of a triangle is 180°)
So, the third angle would be =

Third angle would be = 30°
Now we could use SOH CAH TOA to find the side.
SOH means

Hypotenuse is given as 9√6 and opposite is x
we need to find x

Putting the value of sin 30 as


Cross multiplying

: Answer
Hope it will help :)