Answer
The mass of the air which is ejected should be 12 kg.
Work Step by Step
We can use conservation of momentum to solve this question. Let $m_A$ be the mass of the astronaut and let $m_B$ be the mass of the air. Therefore,
$m_B~v_B = m_A~v_A$
$m_B = \frac{m_A~v_A}{v_B} = \frac{(210~kg)(2.0~m/s)}{35 ~m/s}$
$m_B = 12~kg$
The mass of the air which is ejected should be 12 kg.