х^2 - 2х-3=0
D=(-2)^2 - 4·1·(-3)=16
x1=2+4/2=3
x2=2-4/2=-1
a = 1 , b = -2 , c = -3
D = * a * c
D = 4 - 4 * 1 * ( - 3 ) =
D = 4
x1 =
x2 =
* a * c 
