1) 6х+(3х-2)=14
6х + 3х - 2 = 14
9х = 16
х = 16/9
2) 8у-(7у-142)=51
8у - 7у + 142 = 51
у = -91
3) (6х+1)(3-2х)+4 = -5х-3
18х - 12х^2 + 3 - 2х = -5х - 3
-12х^2 + 16х + 3 = -5х - 3
-12х^2 + 16х + 3 + 5х + 3 = 0
-12х^2 + 21х + 6 = 0
-4х^2 + 7х + 2 = 0
4х^2 - 7х - 2 = 0
D = 49+32 = 81 = 9^2
x1 = 7 - 9 / 8 = -2/8 = -0,25
x2 = 7 + 9 / 8 = 2