3. Matrix Multiply

Implement matrix multiplication for two 2D matrices $A$ and $B$.

Given $A \in \mathbb{R}^{m \times k}$ and $B \in \mathbb{R}^{k \times n}$, compute:

$C_{ij} = \sum_{l=1}^{k} A_{il} \cdot B_{lj}$

The result $C$ has shape $m \times n$.

Example

A = [[1, 2],      B = [[5, 6],
     [3, 4]]           [7, 8]]

Output: [[19, 22],
         [43, 50]]

Constraints

• Inputs are given as lists of lists (not NumPy arrays), but you may convert internally.
• You may assume the dimensions are compatible for multiplication.