Once you declare the type of a variable, you don't need to cast it to that same type. So you can write a=&b;. Finally, you declared c incorrectly. Since you assign it to be the address of a, where a is a pointer to int, you must declare it to be a pointer to a pointer to int.
#include <stdio.h>
int main(void)
{
int b=10;
int *a=&b;
int **c=&a;
printf("%d", **c);
return 0;
}