public String getSecondToLastLetter(String s)

Given a String, return ONLY its SECOND-TO-LAST letter.

Notice in the solution below that you need the second stop parameter!
That is you start at len-2 and stop at len-1!

public String getSecondToLastLetter(String s) {
  int len = s.length();
  String str = s.substring(len-2,len-1);
  return str;
}