Show the worst-case time O(?) for the algorithm

Please use the the tree method and then guess the summation to show how to solve visually.

4) An algorithm is designed that is a combination of insertion sort and merge sort. The algorithm sort the array A[1...n] with the following steps (n is an even number): Step 1) Build n/m sub-arrays Step 2) sort sub-arrays of length m using insertion sort Step 3) Merge sub-arrays -Show the worst-case time O(?) for the algorithm, when n/m sub-arrays have the length m. 4) An algorithm is designed that is a combination of insertion sort and merge sort. The algorithm sort the array A[1...n] with the following steps (n is an even number): Step 1) Build n/m sub-arrays Step 2) sort sub-arrays of length m using insertion sort Step 3) Merge sub-arrays -Show the worst-case time O(?) for the algorithm, when n/m sub-arrays have the length m