E(x)=(7x-3x²)/(1-9x²) - 3x/(1-2x-3x²) × (1+(3x+x²)/(x+3)=
=(7x-3x²)/(1-3x)(1+3x) - 3x/(x+1)(1-3x) × (x+2+3x+x²)/(x+3)=
=(7x-3x²)/(1-3x)(1+3x) - 3x/(x+1)(1-3x) × (3+4x+x²)/(x+3)=
=(7x-3x²)/(1-3x)(1+3x) - 3x/(x+1)(1-3x) × (1+x)(3+x)/(x+3)=
=(7x-3x²)/(1-3x)(1+3x) - 3x/(1-3x) × 1/1=
=(7x-3x²-3x-9x²)/(1-3x)(1+3x) =
=(4x-12x²)/(1-3x)(1+3x) =
=4x(1-3x)/(1-3x)(1+3x) =
=4x/(1+3x)
1-2x-3x²=1+x-3x-3x²=(x+1)-3x(1+x)=(x+1)(1-3x)
3+4x+x²=3+3x+x+x²=3(1+x)+x(1+x)=(1+x)(3+x)
