(a+b)^3=a^3+3a²b+3ab²+b^3
(x+3)^3 =x^3 +9x²+27x +27
(x+1)^3 =x^3 +3x² +3x +1
x^3+9x²+27x+27 -x^3 -3x² -3x -1 =56
6x² +24x +26 =56
6x² +24x +26 -56 =0
6x² +24x -30 =0 :6
x² +4x -5 =0
D=b² -4ac(a=1,b=4,c= -5)
D=16 +20 =36 =6²
x1,x2=(-b+-корень изD)/2a
x1=(-4 -6)/2 = -5
x2=(-4 +6)/2 =1