input: Write a function to determine the index at which a given value is present in an array. If the value is not found, return -1. The function should take an array `arr` and a value `x` as inputs.
  assert find_index([], 5)==-1
  assert find_index([2, 5, 9, 15], 5)==1
  assert find_index([2, 5, 9, 15], 7)==-1
  assert find_index([3, 1, 2], 2)==2
  assert find_index([3, 1, 2], 3)==0
output: def find_index(arr, x):
      for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1