I came out with this solution for Vo:

(V1 - V2) x (Rf/Ri +1).

But its hit or miss when I change the values in a simulator.

How would solve it?

 

Look here at chapter 4 on page 53
http://focus.ti.com.cn/cn/lit/an/slod006b/slod006b.pdf

 

Just guessing, I would think something like this would be correct:

Vo = V1(Rf/Ri + 1) - V2(Rf/Ri)

But then you still have the problem of determining V1 and V2 (V1 being the + input of the opamp).

You have two things labeled V1 on your schematic; bad practice.