Practise IB Physics SL A.2.16 by calculating momentum before and after collisions and applying conservation in isolated systems.
Assuming that the lithium-7 nucleus in (b) is at rest, suggest why, in terms of conservation of momentum, the neutron initiating the reaction must have an energy greater than 2.5 MeV .
2.5 MeV must be converted to mass (in the interaction) / otherwise the products would not be moving;
(to conserve momentum) final products must have total momentum equal to that of incoming neutron (so extra energy is required) / OWTTE;