WA for POLYMUL

Link to Question http://www.spoj.com/problems/POLYMUL/6
Link to my code: http://ideone.com/g5a7aj6

Please provide me a test case where this doesn't work. I got a wrong answer in this, and all of the test cases I can think of are working here. Thank you for your help!

The code:

//multiplication of nth order polynomials
#include <stdio.h>
#include <stdlib.h>
void printpolynomial(int a[], int n)
	{
	int i;
	for(i=n; i>=0; i--)
		printf("%d ", a[i]);
	printf("\n");
	}
void polymult()
	{
	int i, n;
	scanf("%d", &n);
	int *a= (int *) malloc((n+1)*sizeof(int));
	for(i=n; i>=0; i--)
		scanf("%d", &a[i]);
	int *b= (int *) malloc((n+1)*sizeof(int));
	for(i=n; i>=0; i--)
		scanf("%d", &b[i]);
	int *answer= (int *) malloc((2*n+1)*sizeof(int));
	for(i=0; i<(2*n+1); i++)
		answer[i]=0;
	int index_a, index_b;
	for(index_a=0; index_a<=n; index_a++)
		for(index_b=0; index_b<=n; index_b++)
			answer[index_a+index_b]+=(a[index_a]*b[index_b]);
	printpolynomial(answer, 2*n);
	free(a); free(b); free(answer);
	}
void main()
	{
	int t;
	scanf("%d", &t);
	while(t-->0)
		polymult();
	}