Hi,
To derive this equations paulmdrdo applied correctly Nodal analysis at node labeled V1 and V2 to get Vo=-2.5V, and incorrectly at nodes labeled V1 and Vo (as poined out by tyassin).
The correct set of ecuations are:
1.Applied at node V1 and V2:
2mA= V1/5k+(V1-Vo)/10k+V1/20k
V1/20k+Vo/10k=0
2.Applied at node V1 and Vo:
2mA= V1/5k+(V1-Vo)/10k+V1/20k
(V1-Vo)/20k + Iamp_out = Vo/20k
Obviously you cand find Vo form the first set of ecuations. In the second set, you need to know what Iamp_out is.