why we added 97 then minus it?

Question

import java.util.*;
 class multiplicativeCipher{
public static void main(String args[]){
Scanner sc=new Scanner(System.in);
int shift,i,n;
String str;
String str1="";
String str2="";
System.out.println("Enter the plain text);
str=sc.nextLine();
str=str.toLowerCase();
n=str.length();
char ch1[]=str.toCharArray();
char ch3; char ch4;
System.out.println("Enter the key”);
shift=sc.nextInt();
System.out.println();
System.out.println("Encrypted text is");
for(i=0;i<n;i++){
if(Character.isLetter(ch1[i])){
ch3=(char)(((int)ch1[i]*shift-97)%26+97);
str1=str1+ch3;}
else if(ch1[i]==' ' {
str1=str1+ch1[i]; }
}
System.out.println(str1);
 

Answer 1

I think it is used to find a letter from ASCII table. The first lowercase letter in decimal in the table is 'a' => number 97. Then calculating the current number of the letter in decimal and converting it to char. You can see here the ASCII table http://www.asciitable.com/mobile/.