C++ :

#include <iostream>
#include <iomanip>

using namespace std;

int main(){
	int n,m,t,i,a[1000];
	cin>>n>>m>>t;
	for(i=0;i<n;i++){
		cin>>a[i];
	}
	for(i=0;i<n;i++){
		cout<<a[i]<<" ";
		if(m==a[i]){
			cout<<t<<" ";
		}
	}
}

Python :

n,f,b=list(map(int,input().split()))
m=list(map(int,input().split()))
i=m.index(f)
m.insert(i+1,b)
for item in m:
    print(item,end=" ")