Physics help please thanks?

Conservation of momentum tells us that the asteroids momentum, m_a*v_a, must be equal in magnitude to the momentum of superman, m_s*v_s, in the opposite direction. (m_a*v_a = m_s*v_s) A little algebra allows us to set the following ratios equal to each other. m_a/m_s = v_s/v_a So if m_a = 750*m_s, then v_s = 750*v_a. So v_s = 750*120=90,000 m/s.To convert this into mph, you would multiply by 3600 to change seconds into hours and then divide by 1609 to convert meters into miles. 90000*3600/1609 = 2.01*10^5 mph