Nodal analysis: solve the following systems of equations for v1, v2 and v3.
(a) 2v2 − 4v1 = 9 and v1 − 5v2 = −4;
(b) − v1 2v3 = 8; 2v1 v2 − 5v3 = −7; 4v1 5v2 8v3 = 6