Answer
Division by $y$
Work Step by Step
We are given the equation $$3y^2=21y.$$ In the first step each side was divided by $y$, which is an error because $y$ is a variable which can be $0$.
The correct steps are as follows:
$$\begin{align*}
3y^2&=21y\\
3y^2-21y&=0\\
3y(y-7)&=0\\
y(y-7)&=0\\
y&=0\text{ or }y-7=0\\
y&=0\text{ or }y=7.
\end{align*}$$ The roots are $y=0$ and $y=7$.
