Answer 

  double absoluteValue(double u)
  {
    double m;
    if(u >= 0) m = u;
    else m = -u;
    return m;
  }
If I did not ask you to use the solution from the preceding exercise, you could produce a shorter definition as follows. 
  double absoluteValue(double u)
  {
    if(u >= 0) return u;
    else return -u;
  }