How do I find the function rule for an exponential function?

The function values are like this:

x- -2 -1 0 1 2

f(x)- 1 2 4 8 16

I know the answer is f(x)= 4(2)to the x power

I don't get how to come up with the function rule.

I'm teaching from the book Glencoe Algebra 1 1997.

The problem was on a supplemental review I printed out from http://www.mcps.k12.md.us/curriculum...B%20review.pdf

and it was problem #2 c.

The answer key was at http://www.mcps.k12.md.us/curriculum...view%20KEY.pdf

I can't see anyplace in the text that I have that explains how to determine the function rule other than to study the data table or look at the graph. I can't find any instruction or practice on this so any help or a link to someplace to learn and practice finding the function rule would be appreciated.

Brett Taylor

Teacher- Tomlinson Adult Education Center

I am certified in math grades 5-9 and mainly teach adult GED students but I have a few credit math students.

Thanks-