Ответ:

4. 3х²-4х-20=0

D=b²-4ac=(-4)²-4×3×(-20)=16+240=256 (16²)

x1,2=-b±√D/2×a=-(-4)±√256/2×3=4±16/6

x1=4+16/6=20/6=3,33

x2=4-16/6=-12/6=-2

(3,33 ; -2)