If $\begin{bmatrix} x^2 & 3 \\ y & 2x \end{bmatrix} = \begin{bmatrix} 9 & 3 \\ 4 & -6 \end{bmatrix}$, what is $x + y$?

Explanation

1. 01

   $x^2 = 9$ allows $x = 3$ or $x = -3$, but $2x = -6$ leaves only $x = -3$.

2. 02

   $y = 4$, so $x + y = -3 + 4 = 1$.