The scanf call uses the wrong format string. You are reading an int so you need it to be:
scanf("%d", &fahrenheit);
The expression 5/9 is evaluated using integer division. In fact the compiler can work it out at compile time. That expression evaluates to 0.
You need to perform floating point division. For instance:
5.0/9
Or:
5/9.0
Or
5.0/9.0
You just need at least one operand to be a floating point value.
Putting this into your expression, you can write:
c = 5.0/9.0 * (fahrenheit - 32);
and obtain the answer that you expect.
Your printf statement is wrong too. You should enable warnings and let the compiler tell you that. You meant to write:
printf("Here is your %f in celsius!.\n", c);