
This is a question from coordinate geometry. This is a very simple question. You can solve it by using distance formula.
      
First we have to assume the coordinates as following:-
Let the coordinates of A be (8,4) and coordinates of B be (5,1).
      
Further we have to assume:-

     
        

    
        

      
      

      
Here we have distance formula:-

      

      

 

         

      
 

      
This is your final answer.
   
KINDLY VISIT WEB TO UNDERSTAND MORE CLEARLY.