From the reaction, we see that for every 2 moles of Al reacted, 852 kJ of energy is released. So convert the mass given to moles:
moles Al = 48.7 g / (26.98 g/mol)
moles Al = 1.805 mol
So the energy evolved is:
Energy = -852 kJ / 2 mol * 1.805 mol
Energy = -769 kJ