Answer
$v=1.08 \ Mm/s$
$m_2=4 \ u$
Work Step by Step
We know the following equation for elastic collisions:
$v_{1f}=\frac{m_1-m_2}{m_1+m_2}v_{1i}+\frac{2m_2}{m_1+m_2}v_{2i}$
$-8.62=\frac{1-m_2}{1+m_2}(6.9)+\frac{2m_2}{1+m_2}(-2.8)$
$m_2=4 \ u$
We use conservation of momentum to find:
$1(6.9)+4(-2.8)=1(-8.62)+4(v)\\ v=1.08 \ Mm/s$
