[tex] (x+1)(x-1)=x^2 +5x+4 \\ (x+1)(x-1) = x^2+x+4x+4 \\ (x+1)(x-1) = x(x+1)+4(x+1) \\ (x+1)(x-1) = (x+1)(x+4) \\ (x+1)(x-1)-(x+1)(x+4) = 0 \\ (x+1)\big[(x-1)-(x+4)\big] = 0 \\ (x+1)(x-1-x-4)=0 \\ (x+1)\cdot(-5)=0 \big|:(-5) \\ x+1=0 \\ \\ \Rightarrow \boxed{x=-1}[/tex]
