(Where a2 is 'a' squared, b2 is 'b' squared and 2b is 2 times 'b'. I can't figure out superscript on here and pasting from word loses the formatting). The steps are numbered to make any potential answers easier to understand.

1. if a = b

2. a2 = ab

3. a2 -b2 = ab- b2

4. (a+b)(a-b) = b(a-b)

5. a+b = b

6. 2b = b

7. 2 = 1

What can't I see?