What was he writing the letter with? (Make it passive)

What was he writing the letter with?  = With what was he writing the letter?  
এখানে With what – Subject বা Object কিছুই না। 
তাই আমরা বাক্যটাকে প্রথমে ধরে নেই- 
was he writing the letter?
এর Passive হবে- 
was the letter being written by him?
এবার what ও with যোগ করে নেই: 
What was the letter being written with by him?