f(x) means to evaluate the function for a given number x. ("f" is just the name of the function - it could have been named g(x), or z(x) or *(x)... ).
In this case they gave you the number to evaluate this function: -7.
So simply plug the number they want you to use (-7) and perform the arithmetic.
1/(-7) - 12
Use PEMDAS (parenthesis, exponents, multiplication, division, addition, subtraction) in this order to solve it.