hey dude the first thingy u got to do is use
printf ("enter the value of a \n");
and not
printf ("enter the value of a \n",a);

next give main declaration as
void main(void)

and then remove ur return statement.

the correct code is
void main ( )
{
int a, b, c;
printf("enter a number a \n");
scanf("%d",&a);...