I assume you mean something like f(x)=1+k equations or something like that, Guest. Correct me if I'm wrong please ;)
These are kind of like two variable equations; f(x) usually represents y or another variable.
You can replace the f(x) as another variable, like y=1+k to make it easier.
However, if it is f(k), that means you plug in the value of k given.
For example: f(k)=1+k, k=1. This is the same as f(1)=1+1=2.