SInce each question has an alternative so there are 3 ways to answer each of the six questions -
1. 1st Alternative
2. 2nd Alternative
3. No answer
So there are 3 possiblities for each of the 6 questions. Therefore total no. of ways in which a question paper can be answered = 3*3*3*3*3*3 = 729
But in ur case atleast one question must be answered.
The way in which no answer is given to any of the question comes to 1
So in ur case answer = 729-1 =728