Two resistors to create a voltage divider, 2k2 and 1k will get you close, below uses 3 to get a more accurate 3:1 ratio.
Question is if you can accept 7.5V to be translated to 2.5V?
Your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advise on) your project 
See About the IDE 1.x category.