(3х+2.1)(8-2х)=0

24х+16.8-6х²-4.2х=0

-6х²+19.8х+16.8=0

-30х²+99х+84=0

Д=9801-4*(-30)*84=19881=141²

х=-99±141 / 2*(-30)= -0.7; 4

|5х-6|=4

1 случай: 5х-6=4 => 5х=10 => х=2

2 случай: 5х-6=-4 => 5х=2 => х=2/5 => х=0.4