example 6.5 p.108

#1

(v2/r2 + i3) r1 + v2 /. v2 ->  v0 + r3/r4 v0 /. 
      i3 ->  v0/r4
Factor[Simplify[%]]

#2

eq = {vi == (i2 + i3) r1 + v2, i2 r2 == v2, 
        i3 (r3 + r4) == v2, vo == i3 r4}

Solve[eq, vo, {v2, i2, i3}]