K2

a)
$\left(2x+3\right)^2+3x^2=2x^2+3^2+3x^2=5x^2+9$
b)
$\left(3-x\right)\left(3+x\right)-x^2=\left(3^2-x^2\right)-x^2=-2x^2+9$
c)
$\left(x^3+1\right)\left(x^3-1\right)=\left(x^3\right)^2-1^2=x^6-1$
d)
$\left(x+1\right)\left(x-1\right)\left(x^2+1\right)=\left(x^2-1^2\right)\left(x^2+1\right)=\ x^4+x^2-x^2-1=x^4-1$