Thermochemistry problems?
H is enthalpy.  

By definition H = U + PV (where U is internal energy, P is pressure and V is volume).  

For question 1, your work says:


  (250) x (4.184) x (26.26-24.85)
  
  = 1474.86 kJ


Because you failed to show units in your work, you are off by a factor of 1000.

But the main reason your answer is different from the desired answer is you have calculated the amount of heat released by 0.152 g reacting, whereas the desired answer comes from calculating the heat released by 3 moles of A reacting, considering change in enthalpy of the system to be negative when heat is released to the surroundings.  

For question 2:

Answer ii is wrong because the monoatomic forms of hydrogen and bromine are not the standard states of the elements.  Answer iii is wrong because the equation is written for 2 moles being formed rather than 1 mole.