Help~~~~唔識一條數~~~急

Old volume:50*30*12=18000
new volume:50*30*12+5700=23700
new depth=23700/50/30
=15.8cm

the figure shows a rectangular tank of length 50cm, width 30cm and height 20cm. 60% of the tank is filled with water, and then stones of total volume 5700cm3 are put into the tank. If all the stones are totally submerged, find the depth of water in the tank.
Assume h is the depth of water in the tank.
so
50 x 30 h – 50 x 30 x 20 = 5700
1500h = 35700
h = 23.8cm