How do I calculate for change in enthalpy

Calc. change in enthalpy for reaction PCl3+Cl2-->PCl5 given the following

P4(s)+6Cl2-->4PCl3(l) H=-1280. kJ
P4+10Cl2-->4PCl5 H=-1774kJ

I know the answer is -123.8kJ but don't know how to set it up or solve for it :/ I have a test coming up next week and need to figure how to do equations like these

P4 + 6Cl2 -->4PCl3.... H=-1280. kJ ...(1)
P4 + 10Cl2 --> 4PCl5...... H=-1774kJ.....(2)

subtracting (1) from (2) ...

P4 + 10Cl2 - (P4 + 6Cl2) --------> 4PCl5 - 4PCl3 .....H = -1774 - (-1280)

P4 + 10Cl2 - P4 - 6Cl2 --------> 4PCl5 - 4PCl3 .....H = -1774 + 1280 = -494 kj/mole

4Cl2 + 4PCl3 --------> 4PCl5 .....H = -494 kj/mole

dividing by 4 ...

PCl3 + Cl2 -----------> PCl5 ....H = -494/4 = -123.5 kj/mole

PCl3 + Cl2 -----------> PCl5 ....H = -123.5 kj/mole