f(x)= 1/x-12 find f(-7)
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.
Essentially, when a question such as this asks you to find f(any number here) you are replacing x in the equation with the number within the parenthesis. So in this case, f(-7) = (1/-7)-12. You simply place the number value into the x position and solve.