Proof Writing & Problem Solving · Medium problem

A "proof" that $2 = 1$: Let $a = b$. (1) Then $a^2 = ab$. (2) So $a^2 - b^2 = ab - b^2$. (3) So $(a + b)(a - b) = b(a - b)$. (4) So $a + b = b$. (5) So $2b = b$, and $2 = 1$. Which step is invalid?