Hey there!
To find the volume, we simply find the separate volumes of all three cubes and add them. The formula for volume in a cube is s^3 with s being one side. Therefore since all the sides are 6, the volume of one cube is 216 units cubed.
Now, we multiply that by three to get the total volume of the three cubes, to get 648 cubic units- so A.
Hope this helps!