It asks about 45 kcal = 45000 cal. You have the answer for 4.5kcal = 4500 cal.
Also, this answer assumes that the 100g of water are at 100oC already. Note that your answer doesn't need to be as complicated (the 100g part doesn't matter). All you need to do is divide 45000cal by 540 cal/g (which is basically the same thing you did but less work).