The problem is okay. You don't need to know the original depth of the water inside the tank to know how much it rose after the ball is thrown in.

The base tank is 18 by 6 .... I assume in inches..., so if y were the change in depth, then

18*6*y = volume of ball

So,

18*6y = 18pi

y = 18pi / 18*6

y = pi/6 inches -------answer.