Add the following vectors: a) X + Y where X = [10, 4] and Y = [-4, 8]; b) [7, 2] + [5, -1]. Find AB as a column vector if OA = [2, 4] and OB = [3, 2].

$$X + Y = \begin{pmatrix} 6 \\ 12 \end{pmatrix}$$

The sum is $\boxed{\begin{pmatrix} 6 \\ 12 \end{pmatrix}}$.

$$\begin{pmatrix} 7 \\ 2 \end{pmatrix} + \begin{pmatrix} 5 \\ -1 \end{pmatrix} = \begin{pmatrix} 12 \\ 1 \end{pmatrix}$$

The sum is $\boxed{\begin{pmatrix} 12 \\ 1 \end{pmatrix}}$.

2. Find AB as a column vector
Given $\vec{OA} = \begin{pmatrix} 2 \\ 4 \end{pmatrix}$ and $\vec{OB} = \begin{pmatrix} 3 \\ 2 \end{pmatrix}$.

$\vec{AB} = \vec{OB} - \vec{OA}$.

$$\vec{AB} = \begin{pmatrix} 3 \\ 2 \end{pmatrix} - \begin{pmatrix} 2 \\ 4 \end{pmatrix}$$

$$\vec{AB} = \begin{pmatrix} 1 \\ -2 \end{pmatrix}$$

The column vector $\vec{AB}$ is $\boxed{\begin{pmatrix} 1 \\ -2 \end{pmatrix}}$.