What is the mean proportional between 3 and 27?

The mean proportional between 9 + $$3\sqrt{7}$$ and 18 – $$6\sqrt{7}$$ is:​

The mean proportional between 0.09 and 16 is:

The mean proportional between $$\left(9 + 4\sqrt{5}\right) \text{ and } \left(18 - 8\sqrt{5}\right) \text{ is:}$$​