K3

a)
$f\left(x\right)=2x\left(3x+4\right)+1=6x^2+8x+1$
b)
$f\left(x\right)=3\left(2x-1\right)+x\left(-4x+5\right)=6x-3+\left(-4x^2\right)+5x=-4x^2+11x-3$
c)
$f\left(x\right)=-\left(2x-1\right)\left(x+3\right)=-\left(2x^2+6x-x-3\right)=-2x^2-5x+3$
d)
$f\left(x\right)=1-\left(3x^4+1\right)^2=1-3x^8-1=3x^8$