Can´t see what´s wrong?
.
We start with:
$\Large x2 + 6x + 5 = x2 - 1$

Factor both sides of the equation:
$\Large (x + 5)(x + 1)=(x - 1)(x + 1)$

Divide both sides by $\Large (x + 1)$
$\Large (x + 5)=(x - 1)$

Subtract $\Large x$ from both sides

$\Large 5=-1$

Question

Can´t see what´s wrong?
.
We start with: 
$\Large x2 + 6x + 5 = x2 - 1$

Factor both sides of the equation:
$\Large (x + 5)(x + 1)=(x - 1)(x + 1)$

Divide both sides by $\Large (x + 1)$ 
$\Large (x + 5)=(x - 1)$

Subtract $\Large x$ from both sides

$\Large 5=-1$

hartnn · Answer

$x^2+6x+5=x^2−1$

gives you 
6x +5 = -1

6x = -6
x= -1

x+1 = 0


since x+1 =0, you CANNOT divide both sides by x+1

hartnn · Answer

dividing both sides by 0, is prohibited in mathematics

hartnn · Answer

if you divide both sides by 0, you might end up in math jail for breaking the math rules :P