Hi Oleg,

You've zeroed (pun intended) in on the error, but in the words of Maxwell Smart, you've "... missed it by thaaaat much" (picture holding the tips of your index finger and thumb very close together).

There is no rule that you can not multiply by zero. The only rule concerning such a thing is that anything you do either side of the equation, you must do to both sides of the equation. This is a fundamental rule of algebra. Nonetheless, you are extremely close.