Write the system of linear equations, use x,y,z and if necessary w,x,y,z.
Once system is written, use back-substitution to find its solution:

|1 2 -1 0 | 2|
|0 1 1 -2 | -3|
|0 0 1 -1 | -2|
|0 0 0 1 | 3|
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