Open circuit voltage
Converting current source into voltage source

You could read this article.
If the current source confuses you, you could convert it into an equivalent voltage source using the following theory:

http://en.wikipedia.org/wiki/Source_transformation

Then,

The 2K ohm and 4K ohm resistors have zero current flowing through them (open circuit), and hence can be discarded in our voltage calculations. Since we need to find Vo, which is a potential difference, we can assume a reference node (top one) to be at 0V. Thus, the current division would be in the inverse ratio of the resistances in the parallel branches.

Hence, I1 = 15/(15+20) * 17 mA = 7.2857 mA.

Similarly, I2 = 20/(20+15) * 17 mA = 9.7142 mA.

Thus, Vo = I1 * 10 - I2*3 = 72.857 - 29.142 = 43.714 V.

This is your answer, and if you have any doubts, you could go through any of these articles:

http://en.wikipedia.org/wiki/Current_divider