10)

This is a right-angled triangle with angles $$90^\circ$$, $$60^\circ$$, and $$30^\circ$$. The side opposite $$30^\circ$$ is $$y$$. The side opposite $$60^\circ$$ is $$18$$ m. So $$y\sqrt{3} = 18$$, $$y = 18/\sqrt{3} = 6\sqrt{3}$$ m.