To solve these, you must know that ΔH = (sum of bond enthalpies of bonds broken) – (sum of bond enthalpies of bonds formed).
In the (a) reaction, you must break a C-H bond and a Cl-Cl bond. You then form a C-Cl bond and an H-Cl bond. Look up the values of those bonds in your table and calculate ΔH using the relationship I gave.
In (b), you are breaking 2 S-C bonds and 2 H-Br bonds, and you are forming 2 S-H bonds and 2 C-Br bonds.
In (c), you are breaking 1 N-N bond and 1 Cl-Cl bond, and forming 2 N-Cl bonds.
These are really easy problems to solve once you figure out how to recognize which bonds are broken and which are formed.
Hope that helps…