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almaren
answer is 9 root3
this is because it is one of the special triangle: with special ration of 2x:root3x:1x
you could plug values into the sin formula as well.

9 cm is the smallest length since it is across from the 30° angle. so there fore 9 is 1x
x is 9.

root3x = 9root3
As, it is a right angled triangle
Taking, 
tan 60 = x/9 [Perpendicular/Base]
∴ √3 = x/9
x=9√3
or x= 9*1.732
∴ x= 15.588