Approach 1. Actually construct the sequence \(B\)

After receiving \(N,K,X\), and the sequence \(A\), we can insert \(X\) after the \(K\)-th element of \(A\).
For example, if you use the list type of Python, you can use the insert method.
For other languages, please refer to articles on array handling or language reference of the language.

Sample code (Python):

n,k,x = map(int,input().split())
a = list(map(int,input().split()))
a.insert(k,x)
print(*a)

Regarding the print(*a) part, one can prepend * before the list name to print it without unnecessary brackets.

Approach 2. Instead of actually constructing the sequence \(B\), print it sequentially

Actually, we can properly output the result without actually constructing the sequence \(B\) nor even storing the array \(A\).
Specifically, receive each element of the sequence \(A\) and directly print it. After printing the \(K\)-th element, print \(X\). Finally, receive and print the remaining elements of \(A\).

Sample code (C++):

#include<bits/stdc++.h>

using namespace std;

int main(){
  int n,k,x;
  cin >> n >> k >> x;
  for(int i=0;i<n;i++){
    int a;
    cin >> a;
    if(i!=0){cout << " ";}
    cout << a;
    if(i==(k-1)){
      cout << " " << x;
    }
  }cout << "\n";
  return 0;
}