Пошаговое объяснение:
1) -y +0.5x +2y =0.3
y = - 0.2
x = 0.7
2) -x-6.9 -8x = 83.1
-9x = 90
x = -10
y = 3.1
3) 21x + 0.9 - x = -15.1
20x = - 16
x = 0.8
y = 0.1
