You may want to review evaluating expressions first.

A function is an expression which can have *x* in it. It is
written like 3*x* −5. To evaluate a function for a
given value of *x*, you put that value in wherever you
see *x*. In the above example, if *x* = 5 then the
function comes out to 3 * 5 − 5, which is 15 − 5 or
10.

If the function doesn't have *x* in it, then the value
of *x* doesn't matter. Just work out the expression and
you're done.

Be careful if the value given for *x* is negative or a
fraction. It might make it easier for you to put it in
parentheses when you put it in. For example, in the example of
3*x* −5, if *x* = −1, you can put it in as
3(−1) − 5, which is −3 − 5, or −8.
If you didn't use parentheses, you might get 3 − 1 −5,
which is 2 −5 or −3. This would be wrong.