You may want to review evaluating expressions first.
A function is an expression which can have x in it. It is written like 3x −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 3x −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.