Arithmetic question

Using Arduino Programming Questions
1

Hello !
I would like to make some arithmetic, not sure how to do it. Here is my code :

      case 'H':         //stabtrim test
      delay (11);
      stabtrim = "";
      stabtrim += getChar();
      stabtrim += getChar();
      stabtrim += getChar();
      stabtrim += getChar();
      
      if (stabtrim != stabtrimold){
         lcd.setCursor(0, 0);
         delay (11);
         // newstabtrim = (0.1289 x stabtrim) + 4.1
         lcd.print("Trim : " + stabtrim);
         stabtrimold = stabtrim;
      }        
      break;

So, as you can see in comments, I have to change the stabtrim value with the formula to display newstabtrim. Any idea how I can do it ?

2

Go here first and read. Pay particular attention to the section on mathematical operators.

You've almost got it, but you're using the wrong symbol for multiplication.

3

Why type is stabtrim?
What does getChar() return? You appear to be adding characters. Unless stabtrim is a String, in which case you haven't a hope in hell of multiplying it by anything.

4

Oh I missed that part.

Yet more evidence that one should post complete code.