String value = ......;
char last = value.chatAt(value.length()-1);
or
String last = value.substring(value.length()-1);

just check the Javadoc of java.lang.String for all other methods.

To start with:

char c = String.getCharAt(String.length-1);
if (c == 'c'){
  //equation 1
}
else{
  if(c == 'f'){
    //equation 2
  }
  else{
    //and now for something completely different
  }
}

Then you need to find a way to remove the letter and convert the numbet
to an int/long/float/double, whichever best suits your needs.

See StringBuffer.deleteCharAt(int index), String.substring(int begin,
int end), Integer.parseInt(String), Long.parseLong(String),
Double.parseDouble(String), for example.
THe definitions for these are easy to find at <http://java.sun.com> or
the documentation delivered with the JDK. (These will doubtlessly answer
many of your future questions as well.)