To multiply the matixes, we'll look at their elements
[tex]A=\begin{bmatrix}{A_1} & {A_2} & {A_3} \\ {} & {} & {} \\ {} & {} & \end{bmatrix},\text{ B=}\begin{bmatrix}{B_1} & {} & {} \\ {B_2} & {} & \\ {B_3} & {} & {}\end{bmatrix}[/tex]In order to get AB, we simply use the following formula
[tex]AB=A_1\cdot B_1+A_2\cdot B_2+A_3\cdot B_3[/tex]In this case
[tex]AB=3\cdot1+4\cdot2+5\cdot3+6\cdot4=3+8+15+24=50[/tex]Since both matrixes have 4 elements. Thus
[tex]AB=50[/tex]