State in words the next elementary row operation that should be performed on the system in order to solve it$.[$More than one answer is possible in $($a$).]$

a$.$

$x_1+4x_2-2x_3+8x_4=12$

$x_2-7x_3+2x_4=-4$

$5x_3-x_4=7$

$x_3+3x_4=-5$

b$.$

$x_1-3x_2+5x_3-2x_4=0$

$x_2+8x_3=-4$

$2x_3=3$

$x_4=1$