The rhombus can be split into two isosceles triangles with side 9cm and vertex angle 55°.
If one of the triangles is split into two congruent back to back right triangles, the vertex angle is bisected=27.5°. The height h is given by h/9=cos27.5, so h=9cos27.5=7.983cm approx.
Doubling this height gives the height of the rhombus=15.966cm approx.