Thevenin in diode circuit
First, ask yourself if the diode is forward biased. You can tell this by shorting the diode and seeing which way the current flows. In this case, the diode is in fact forward biased.

Now, with the diode forward biased, what it its voltage drop? Since this is an ideal diode, that should be easy. So, can you treat the two 20k resistors as if they are in parallel? If so, can you derive the voltage at the junction of R1 and R3? How about D1 and R2? Does that solve your problem?