A function is a rule that gives exactly one output for each input. In $f(3)$, the 3 is the input and the value you get after applying the rule is the output.

To evaluate a function, replace every $x$ with the given input. Use brackets for negative inputs, so $h(-4)=2(-4{)}^2-6$, not $2-4^2-6$.

Function notation is the language used for inverse and composite functions too. Once substitution is secure, you can start undoing rules with $f^{-1}(x)$ and chaining rules with $g(f(x))$.