Solve the following SLE with the Gauss algorithm. Mark the pivot in every step.

a$)$

$2x+2y+2z=8$

$3x+y-z=4$

$-2x+3y+z=17$

b$)$

$x_1+x_3=1$

$2x_1+2x_2=4$

$2x_1+2x_3=2$

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c$)$

$2a+b+c+2d=12$

$3b+d=11$

$2d+4b+a+3c=17$

$a+b+c+d=8$