What you need to do is calculate the volume of the Box. With the box being the volume of the piramid added to the prism surrounding the box with volume 15.
If you want to calculate it yourself stop reading here, because I will work it out now.
Volume box = a*b*c
Volume prism = 15
Volume pyramid = (b * c * a)/3
The prism has the volume of the box minus the pyramid so
