Q:

How to print part of words in string by input using array in c?

The C program below print the third until last character of 80 words strings
#include <stdio.h>

int main(void) {
  char a[] = "computer";
  char start = 3;
  char length = 7;

  printf("%.*s\n", length, a + start);

  return 0;
}

the output :
puter

how to write in c code for print "n" last character of string
ex:

string : "this computer"
n inputed by scanf ex: 5

it will print to console
"puter" 

A:

Change   
char start = 3;
char length = 7;

to  
int start;
int length;

and do as follows; I used a pointer in this program.
#include <stdio.h>
#include <string.h>

int main(void) {
    char a[] = "this computer";
    int start;
    int length;
    char *ptr = a; // Initializing ptr with first element of string literal.

    length = strlen(a); // Calculating the length of string literal
    scanf("%d", &start);

    if(start > length)
        start = length;

    ptr += length - start; // Adding the desired last number of elements/characters to be printed to the address ptr points to. 

    printf("%s\n", ptr);

    return 0;
}

Input:  
5  

Output:  
puter

