If volume is 18,000 gallons and depth is 3.8 feet how do I find length and width?

Well, the only way you could find the length and width given these values is if the length and width are equal. Otherwise you have two unknowns in the equation as opposed to one. If Length = Width, You could take:

Volume = Length*Width*Height and rewrite it as:
Volume = Length*Length*Height

And then substitute the appropriate values and solve it. Does the problem specify that the base of the object is a square or something along those lines?

it is a rectangle thank you for asking

Just remember the conversion between gallons and cubic feet. I found this:

1 cubic foot = 7.48051945 US gallons

1 US gallon = 0.133680556 cubic feet