line 3.
it's a proof, forget the equals, do it left side right side.
line 2. LS
a^2-b^2
line 3. RS
(a+b)(a-b)
Simplify this, let both a and b = x we get
(2x)(x-x)
(2x)(0)
(0)
so on line 4 where it goes to (a+b) [ or (2x) as we've now got it] that's not a valid simplification...