6. x +2y =4, x =4 -2y
3(4 -2y) -4y =7
12 -6y -4y =7
-10y =7 -12, -10y = -5
y = 5/10 =0.5
x =4 -2 *0.5, x=4 -1
x =3
проверка
x +2y =4, 3 +2*0.5=4, 4=4
3x -4y =7, 3*3 -4*0.5 =7
9 -2 =7, 7=7
ответ: x =3, y =0.5
7. (2 -x)² -x(x +1.5) =4
4 -4x +x² -x² -1.5x =4
-4x -1.5x =4 -4
-5.5x =0
x =0