Help, I need to answer this question for a home test

1. Given an array of integers A- [a1,a2,... ,an], suppose that there exists an (unknown) index k such that the subarray [a1,... ,ak is sorted in strictly increasing order, and the subarray [ak,... ,an] is sorted in strictly decreasing order. Design an optimum algo- rithm to solve this problem, and analyze its running time