Answer
$\vec{v_{f2}}=+7.5m/s$
Work Step by Step
At the start, 2 disks act as a system moving to the right with $\vec{v}_0=+5m/s$. After the spring is released, disk 1 is pushed leftward and comes to a halt after a while, so $\vec{v_{f1}}=0$. We need to find $\vec{v_{f2}}$
The principle of conservation of linear momentum states that $$(m_1+m_2)\vec{v_0}=m_1\vec{v_{f1}}+m_2\vec{v_{f2}}=m_2\vec{v_{f2}}$$ $$\vec{v_{f2}}=\frac{m_1+m_2}{m_2}\vec{v_0}$$
We have $\vec{v_{0}}=+5m/s$, $m_1=1.2kg$, and $m_2=2.4kg$. Therefore, $$\vec{v_{f2}}=+7.5m/s$$
