9)

This is a right-angled triangle with angles $$90^\circ$$, $$30^\circ$$, and $$60^\circ$$. The side opposite $$30^\circ$$ is $$y$$. The side opposite $$60^\circ$$ is $$20$$ cm. So $$y\sqrt{3} = 20$$, $$y = 20/\sqrt{3} = 20\sqrt{3}/3$$ cm.