#include<stdio.h>;
void main()
{
    int Number;

    printf("Enter a Number:");
    scanf("%d",&Number);

    if(Number%3 == 0 || Number %6 == 0)
    printf("Enterd Number is either Multiple of 3 or 6 :");
    else
    printf("No the enterd Number is Not Multiple of 3 as well as 6 :");

}

OUTPUT : 1.

Enter a Number:21
Enterd Number is either Multiple of 3 or 6 :

2.

Enter a Number:22
No the enterd Number is Not Multiple of 3 as well as 6 :

as you can see in this code in OUTPUT 1    21 is multiple of 3 so If will run but in output 2   22 is nither multiple of 3 and 6 so else will run.