We already know that it should be complete BST with 4 as root and position of every element is fixed. Now only the order of element could be changed only like : 2 always come before 1 and 3 but 1 and 3 in any order Similarly goes for 6 comes before 5/7.

Now we can pick any one elment from 2/6 first say 2 : then need to pick order for left 5 elments - - - - -

As 6 should come before 5/7 we can pick this fixed order of 3 from 5 positions in 5C3 ways and multiplying 2! for 5/7 arrangement . Finally multiplying the 2! for arrangement of 1/3 .

Hence , total ways = 5C3 * 2 * 2 = 40.

Similarly total ways if pick 6 first = 40.

Hence , total ways finally = 40 + 40 = 80.