In alpha decay a helium-4 nucleus is emitted, which has two protons and two neutrons. (Its symbol is 42He.) Thus the initial nucleus loses two protons -- its atomic number is reduced by 2. It loses 4 particles altogether and so its mass number is reduced by 4.
In question 7, 254100Fm (fermium) decays to the element with atomic number 98 and mass number 250. The Element Tables shows that element number 98 is californium, chemical symbol Cf. So in addition to the helium-4 nucleus, we also get 25098Cf.
Similarly for question 9: The initial nucleus is polonium-210, with atomic number 84. After alpha decay we have a nucleus with atomic number 82 and mass number 206. This nucleus is lead, 20682Pb.