Let $A=\left[\begin{array}{rrr}1 & 2 & 1 \\ 0 & 5 & 1 \\ 2 & -1 & 4\end{array}\right] .$ Suppose a row operation is applied to $A$ and the result is $B=\left[\begin{array}{rrr}1 & 2 & 1 \\ 0 & 10 & 2 \\ 2 & -1 & 4\end{array}\right]$
(a) Find the elementary matrix $E$ such that $E A=B$.
(b) Find the inverse of $E, E^{-1},$ such that $E^{-1} B=A .$