Polynomial equations word problem(screenshot included)

If the width is 3 less than the height, we can say that the width is x, and the height is x+3. As the tank is square based, we can say that the length is also x. If the tank has a capacity of 20, this is the volume of the tank. Using the formula for volume, we get (x)(x)(x+3)=20. We multiply this out to get x^3+3x^2=20. If we think this one through, we can realize that 2 works for x, as 8+12 = 20. This means that the dimensions are 2 by 2 by 2+3, or 5.