19a-7b+12, если 5a-8b+2 / 8a-5b+2 = -3;

Пусть 5a-8b+2 = k(8a-5b+2). Тогда k = -3.

5a-8b+2 = -3(8a-5b+2)

5a-8b+2 = -24a+15b-6

29a - 23b + 8 = 0