Help with analyzing a Wheatstone Circuit using Node-Voltage Analysis

[Temp0]

Homework Statement

Calculate the resistance of R3 so that Vab = Va - Vb = 0, assume Rth is 75Ω.

Here is a picture of the circuit: http://i.imgur.com/zsQBYJx.jpg

Homework Equations

The Attempt at a Solution

Never seen this circuit before, don't know where to begin. I assume I have to analyze the voltage at each node and set up a system of equations which can allow me to calculate R3 based off of the values that I already know, but I'm not sure how to begin. Thanks in advance for any hints.

 

[gneill]

You need to show some attempt. Why not start by writing the node equations for Va and Vb?