Consider the integral matrix
$$R = \left(\begin{matrix} 2 & 4 & 6 & -8 \\ 1 & 3 & 2 & -1 \\ 1 & 1 & 4 & -1 \\ 1 & 1 & 2 & 5 \end{matrix}\right).$$
Determine the structure of the abelian group given by generators and relations.
$$A_r = \{a_1, a_2, a_3, a_4 | R \circ \vec{a} = 0\}$$
I know you have to row/column reduce the matrix however am unsure what to do next.