I'm not entirely sure on how to work out the answer but this may point you in the right direction :

1st calculate the amount of energy the oxidation of one mole of hydrogen produces
2nd calculate how many moles of hydrogen would be needed to meet the demand of 10^17kJ
3rd calculate how much water would be required to meet the demand of that many moles of hydrogen
4th calculate the volume of water that is equivalent to then work out the decrease in the sea level