{x+7=2y+1 ⇔ x-2y=1-7 ⇔ x-2y=-6
{3x-y=7 /*(-2)
{x-2y=-6
{-6x+2y=-14 (+)
x-6x-2y+2y=-6-14
-5x=-20 /:(-5)
x=4
x -2y=-6
4-2y=-6 /-4
-2y=-10 /:(-2)
y=5
Solution: (4; 5)
