Solve the following system of equations for the x values:

a) 2x1 + 3x2 - 4x3 = 12

4x2 + 2x3 = 1

3x1 + 2x2 + x3 = -4
 

b) 2x1 + 3x2 - 4x3 + x4 = 0

-2x1  + 4x2 + 2x3 - 10x4 = 3

2x2 + x3 - x4 = 4

x1 + x3 + x4 = 12