You can find the solution in the following figure.  In your case, the RL (in the figure below) is infinity, and the RS (in the figure below) is 2Rss.  You can replace these terms, and the results will be the same.
This link may help you also for other problems (Solution in page 12).

http://aries.ucsd.edu/NAJMABADI/CLASS/ECE65/12-W/NOTES/Amp-2.pdf 

[![enter image description here][1]][1]

  [1]: https://i.sstatic.net/vJeGP.png