Решение
3x + 4y = - 1
2x - 5y = 4
15x + 20y = - 5
8x - 20y = 16
складываем
23x = 11
x = 11/23
x = 11/23
3 * (11/23) + 4y = - 1
x = 11/23
4y = - 1 - 33/23
4y = - 1 - 1 (10/23)
4y = - 2 (10/23)
y = - (56/23) : 4
y = - 14/23
Ответ: x = 11/23 ; y = - 14/23