Let $a + b = c$ be the basic equation.

Then, both sides multiply with $(a+b)$.
$(a + b)^2 = c (a+b)$.
$a^2 + 2ab +b^2 = ac + bc$.

Move $ab$ from left to right; and $ac$ and $b^2$ from right to left.
$a^2 + ab - ac = bc - b^2 -ab$.

Factoring …
$a (a+b-c) = -b (a + b -c)$

Removing $(a+b-c)$
And we have:
$a = -b$.

Oops. It contradicts with our basic equation.

Obviously something is wrong here.

Guess what is the mistake here? It is a primary school’s question in fact 🙂