given an array arr, it’s guranteer that each number in arr appears exactly twice. define a function f(x), says that the indecies of x is i and j, the value of f(x) can be obtained by following

the steps:
1. Find the index k of the first occurrence of x in arr.
2. Find the index l of the last occurrence of x in arr.
3. Compute f(x) = (k+1) * (l+1)

Here's the code for the function in Python:

def f(arr, x):
    k = arr.index(x)
    l = len(arr) - arr[::-1].index(x) - 1
    return (k+1) * (l+1)

The `index` method returns the index of the first occurrence of the element in the array, and the `[::-1].index` method returns the index of the last occurrence of the element in the array, by reversing the array and applying the `index` method again. The `len` function is used to get the length of the array.