[tex](x+1)^2-(2x+1)^2=\\\\=(x^2+2\cdot x\cdot1+1^2)-((2x)^2+2\cdot2x\cdot1+1^2)=\\\\=(x^2+2x+1)-(4x^2+4x+1)=\\\\=x^2+2x+1-4x^2-4x-1=\\\\=x^2-4x^2+2x-4x+1-1=\boxed{-3x^2-2x}[/tex]
