x/y = 5/7 => 7x=5y
x= 5y/7
3x-2y=3 => 15y/7 - 2y=3 ( aducem la acelasi numitor )
15y/7 - 14y/7 = 21/7 ( inmultim cu 7 ca sa ne scapam de numitor)
15y-14y=21 => y=21
Apoi, te duci in prima ecuatie si inlocuiesti => 7x=5*21
7x=105
x=15
Proba : 3*15 - 2*21 = 3
45-42 = 3
3 = 3
