f(x+1)=x²+2x+1-4x-4+3
= (x+1)²-4(x+1)+3
∴f(x)=x²-4x+3
f(3-4x)=(3-4x)²-4(3-4x)+3
=9-24x+16x²-12+16x+3
=16x²-8x