With calculating bond energies.
the formula is c3h8 + br2 --> c3h7br +hbr
c-h, 414; c-br, 276; br-br, 193; h-br, 364.

You must multiply the bond enthalpy by each broken or formed bond.
The C - H bond = 414k Joules/ mol, but that is only applicable per bond.
The C3H8 molecule has 8*C-H bonds that have to break. You have to account all the bonds that break and form to get the DH for the reaction.
Because of the Lewis structure of Propane C3h8 you only use 8 in c3h8 has 3-C and 8-C.