input: Write a function named find_index which takes an array named arr and a value named x. The function should return the index at which x is present in arr. If x is not present in the array, the function should return -1.
  
  assert find_index([2, 5, 9, 15], 5) == 1
  assert find_index([10, 20, 30], 30) == 2
  assert find_index([4, 8, 12, 16], 7) == -1
  assert find_index([1, 1, 1, 1], 1) == 0
  assert find_index([], 5) == -1
output: def find_index(arr, x):
      try:
          return arr.index(x)
      except ValueError:
          return -1