If the vector $\vec{b} = 3\vec{j} + 4\vec{k}$ is written as the sum of a vector $\vec{b_1}$ parallel to $\vec{a} = \vec{i} + \vec{j}$ and a vector $\vec{b_2}$ perpendicular to $\vec{a}$, then $\vec{b_1} \times \vec{b_2}$ is equal to: