Good. Nxb4 is the correct answer. 

Diagram 1 shows the position in the question 

Diagram 2 shows the position after black plays Nxb4  The bishop has been taken so the fork is stopped. 

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In diagram 3 the position is a little different. 
                                        
  How can black escape this time?

Qd4+      

or  Rd8
 

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