If you want a random location, you can use random().

If you want it to be specifically either 100 or 300 use the following code:

1. int xpos = 100 + int(random(2))*200;

Explanation:
Always start with 100. Random will return a value between 0-2. Int will turn this value into 0 or 1. So it's either 0*200 or 1*200. The first is 0, the second is 200. Making the total either 100 or 300. Voilà.