input: Write a function to take an array and a value as input and return the index at which this value is present in the array. If the value is not found in the array, return -1.
  assert find_index([2, 5, 9, 15], 5)==1
  assert find_index([2, 5, 9, 15], 15)==3
  assert find_index([2, 5, 9, 15], 3)==-1
  assert find_index([2, 5, 9, 15], 2)==0
  assert find_index([], 5)==-1
output: def find_index(arr, value):
      try:
          return arr.index(value)
      except ValueError:
          return -1