When n/7th element is chosen as pivot then array wiil be divided into two subparts having size n/7 and 6n/7. Time for doing this will be $O(n^2) $ as given, Recurrence relation can be written as:
$T(n)=T(\frac{n}{7})+T(\frac{6n}{7})+O(n^2)$ // can be solved using recursion tree also
For upper bound we can write as
$T(n)=2T(\frac{6n}{7})+O(n^2)$
Apply master's thorem,
T(n)=$O(nlog_{7/6}n)$