Answer:
m = (2 - 5) / (2 - 0) = -3/2 = -1.5
 
        
             
        
        
        
You need more information.  If you had the bmi's of every man between 20 and 29 you could get an exact answer.  If you had a good sample you could get a good estimate of this answer.
        
             
        
        
        
-2 " -1,5 : -3-2 " -4/3 " -1 " 0 " 1 " 3/2 " 2
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