How to solve a f x equation?

Answer 1

I guess you mean an equation written in function notation, for example:

f(x) = 3x + 2

For the purposes of doing math, it may help to think of this as an alternative way of writing, for example (using the function above):

y = 3x + 2

It isn't clear from the question what you want to "solve", but if you write the equation this way, it might become a bit less confusing.