Explanation:
Arranging in ascending order 
2 , 4 , 7 , 9 , 11 , 15
No of data (N) = 6
Now 
Position of median 
= (N+1)/2 th item 
= ( 6 + 1) /2 th item 
= 7/2 th item 
= 3.5 th item 
Exact Median 
= 3 + 0.5 ( 4th term - 3rd term) 
= 3 + 0.5( 9 - 7)
= 3 + 0.5 * 2
= 3 + 1
= 4 
Hope it will help :)