I need help on this circuit problem? kirchoff/node voltage method

[From: ] [author: ] [Date: 12-09-13] [Hit: ]

http://oi50.tinypic.com/1zh2vch.jpg

thanks guys

a) Va-Vb
b) (Va-Vb)/R1
c) Vs*I1. The power delivered by the source is positive.
d) I2=Vb/R2 and I3=Vc/R4

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