The water level in the tank drops 6" in two days, that is, 3" per day. 17 days is 12 days after the level was 57", so the level will drop a further 36". The depth after 17 days is 57-36=21".
Another way to the solution is to work out the depth of water at he start: 63+9=72". After 17 days the level drops 17*3=51"; 72-51=21".