Sedra and Smith Example 3.2
You stated that the diodes are ideal.  Therefore, if D1 is conducting, the voltage across it is 0 by definition of a ideal diode.  If the voltage across it is 0, then the voltage on both sides is the same, and point B must be 0 V since the other side of the diode is at 0 V (by definition of ground).

So, the question comes down to whether D1 will conduct.  To see if it will, pretend it is open and see what the voltage across it would be.  You have a voltage divider with 10 k&Omega; as the top resistor and 5 k&Omega; as the bottom resistor.  The voltage on the bottom resistor will be 1/3 the total, which is 6.7 V in your case.  Since the bottom is at -10 V, the output of the divider will be -3.3 V absolute (relative to ground).  This would forward-bias D1, so we can conclude it is conducting and the voltage accross it 0.