If $3\begin{bmatrix} 2 & x \\ 1 & 0 \end{bmatrix} + \begin{bmatrix} y & 1 \\ -1 & 4 \end{bmatrix} = \begin{bmatrix} 4 & 7 \\ 2 & 4 \end{bmatrix}$, what is $x + y$?

Explanation

1. 01

   The left side is $\begin{bmatrix} 6 + y & 3x + 1 \\ 2 & 4 \end{bmatrix}$.

2. 02

   So $6 + y = 4$, giving $y = -2$, and $3x + 1 = 7$, giving $x = 2$. Then $x + y = 0$.