Implementing a Boolean function with NOR gates Implement f(a, b, c, d) = Σ m(3, 4, 5, 6, 7, 11, 15) as a 2-level gate circuit
  (a) Using OR gates and NOR gates.
  (b) Using NOR gates only.


I have found that F=ab+d using Karnaugh map.
I have also been able to construct the logic expression using OR gates and NOR gates as in part a of the question.

Can you please provide me some idea on how to construct the logic expression ab+d using NOR gates only?

Please do not say this is a homework help question and close this thread.
I'm not asking to do the entire homework, I have only asked for some idea on how to do part b.
Hint: you know it's a two-level circuit using only NOR gates. There is only one such circuit:

  OUT
   |
  NOR
  / \
 |   | 
NOR NOR
/ \ / \
? ? ? ?


Can you now figure out where to place a, b and d to correctly construct the circuit? Since you have to repeat (at least) one input, does looking at symmetry help?