Two carts collide on a low-friction track. Cart 1 has an initial momentum of \((+10 \mathrm{~kg} \cdot \mathrm{m} / \mathrm{s}) \hat{i}\) and a final momentum of \((-2.0 \mathrm{~kg} \cdot \mathrm{m} / \mathrm{s}) \hat{i}\). If cart 2 has a final momentum of \((-6.0 \mathrm{~kg} \cdot \mathrm{m} / \mathrm{s}) \hat{i}\), what was its initial momentum?