2x+y=8, y=8-2x;
y+3z=5, z=⅓(5-y)=⅓(5-8+2x)=⅓(2x-3);
z+2w=15, w=½(15-z)=½(15-⅓(2x-3));
w+3x=9, ½(15-⅓(2x-3))+3x=9, 15-⅓(2x-3)+6x=18, 45-2x+3+18x=54, 48+16x=54, 16x=6, x=⅜.
w=9-9/8=63/8, y=8-¾=29/4, z=15-63/4=-¾.
CHECK
2x+y=¾+29/4=8, y+3z=29/4-9/4=5, z+2w=-3/4+63/4=15, w+3x=63/8+9/8=9. All correct.
(w,x,y,z)=(63/8,3/8,29/4,-3/4).