7)

This is a 30-60-90 triangle. The side opposite $$30^\circ$$ is $$y$$. The hypotenuse is $$2y$$. The side opposite $$60^\circ$$ is $$y\sqrt{3}$$. We are given the side opposite $$60^\circ$$ is $$4$$ cm. So $$y\sqrt{3} = 4$$, $$y = 4/\sqrt{3} = 4\sqrt{3}/3$$ cm.