#include<stdio.h>
#include<conio.h>
main()
{
 int a[5]={1,2,3,4,5},b[4]={-6,-7,-8,-9},i,j=0;
for(i=5;i<=5+4-1;i++)
{
     
     a[i]=b[j];
     j++;
}                     
for(i=0;i<9;i++)
printf("a[%d]=%d \t",i,a[i]);

getch();
}

To merge to arrays ( Note the question has no mention of 
sorting the array elements, hence the program below just 
merges two arrays)

int array1[5] = {1,2,3,4,5};
int array2[7] = {6,7,8,9,10,11,12};
int i;
int merged[(sizeof(array1)+ sizeof(array2))/sizeof(int)];
memcpy( merged, array1, sizeof(array1));
memcpy( (merged+5), array2, sizeof(array2));

for(i=0;i<(sizeof(merged)/sizeof(int)); i++)
{
printf("%d\n",merged[i]);
}