need help with  O/N 2010 variant 23 q6b (ii)  
Dark fringe, destructive interference, out of phase, 180
o2. Superposition can cause the amplitudes to add up,( Crest Crest), or get subtracted. 
  Max amplitude = 2+1.4 , min = 2-1.4 
   3.4, 0.6
   I = kA^2
   Ratio = k*3.4^2 / k*0.6^2 = 3.4^2/0.6^2 = 32