First, take common sense and apply it here. The problem doesn't specifically tell you what the shape ACTUALLY is, but we can also apply what we know about geometry here, as well as what we know in general. Fish tanks most commonly come in 2 shapes: Square's and Rectangles. Fish tanks are what we call "3D" objects (meaning it's just not flat like a piece of paper", and the shapes are known as "Cubes and Rectangular Prisms".
Well, we know that squares/cubes have to have even sides. The problem states the tank is 5(l) by 2(w) by 2(h). This means it CAN'T be a cube, thus meaning it's a rectangular prism.
Now, to calculate volume, we have to know the formula. For this, the formula for volume is as follows: V=lwh This means Volume=Length*Width*Height
So, do replace the words with what you know you already have. Let's make a list: Length=5 Width=2 Height=2