first we do a binary search on the average and let it be x
we decrease x from all of the array elements and if there exists a sub array with lengh more than k whose sum is more than zero then we can say that we have such a sub array whose average is more than x other wise we can say that there doesnt exist any such sub array
how to find out if there is a sub array whose sum is more than zero and its length is more than k? we can say that a sub array [l, r) equals sum[1, r) — sum[1, l) so if we get the partial sums and fix the r of the sub array we just need an l which sum[1, r) >= sum[1, l) and l <= r — k this can be done with partial minimum of the partial sums