Infinite Sum of a Geometric Series

I am having a bit of difficulty in determining the sum of an infinite geometric series:

Find the sum (assume http://math.colorado.edu/webwork2_fi...66c230c7f1.png):

I was shown that the sum of this series is equal to the following when http://math.colorado.edu/webwork2_fi...66c230c7f1.png:

Where 2 is the first value. But I thought that since this series was increasing by a common ratio of 2x that it really depends upon how small or large x really is. This is where I am stuck. I am just not sure how to go around the lack of a definitive x value. I appreciate the help.