Implementing a Boolean function with NOR gates
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?