12. Rezolvați prin metoda substituţiei sistemele:

a) 2x - 3(y-2) = 19
3(x-y) + 2(x+2y) = 7

b) -4x + 3(x+2y) = -15
3(x+y) - 2(2x-y) = -13

c) 2(2x+3) - 3(y-1) = -1
3(2x+y) - 2(x+y-3) = 4

d) 3(x+y-2) = 4(2x-y+3) + 1
2(2x-3y+1) = 3(3x-4y+5)+4​