Evaluate f(2) where f(x) is a given function. This is done by replacing every occurence of x in the formula defining the function with the given value and then doing the calculations.
Evaluate f(s + t) where f(x) is a given function. This is done by replacing every occurence of x in the formula defining the function with the given expression and then doing the necessary algebra.
The domain of a function is the set of all possible real values for which the function is defined. Functions are undefined, for example, where there would be division by zero or evaluation of a square (or more generally, even) root of a negative number.