Since this is a homework question, you have to use the template that requires you to show what you have done. So start with a) and tell us what you have done to solve the problem, what principles and equations apply etc. Hint: just think of the problem as it says: two masses m1 and m2 connected by a spring of length l and spring constant k whose motion is viewed from the centre of mass frame but where v is the relative speed of m1 and m2.