Step 4: analysis (almost , I think)
this one is almost like the candidate elimination i showed you in the previous step(its called deriving certainty from uncertainty).
Take a look at the picture below.In the center region. There is only two possible places for the 7(and 2). There is three places for the seven in the middle right region , but can not be in the middle ones because 7 is reserved by the center region thus there is only one place let for the seven in the middle right region.