Step-by-Step Solution
Key Concept: Matrix equations can be solved by equating corresponding entries and forming a system of constraints.
For matrices $A = \begin{pmatrix} 4 & 1 \\ -9 & -2 \end{pmatrix}$ and $B = \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix}$, verify that $a + d = 2$ and $\frac{c}{b} = -9$ where entries follow from matrix relationships.
Correct Answer: 3