令a=1+2x
x=(a-1)/2
f(a)=[(a-1)/2]^2-4(a-1)/2-1
f(3-4x)=[(3-4x-1)/2]^2-4(3-4x-1)/2-1
=(1-2x)^2-2(2-4x)-1
=4x^2-4x+1-4+8x-1
=4x^2+4x-4