I'm stuck with my homework in the following task:

a rectangular parallelepiped ( b*b*h ) holdsdm^3.K

Show that the total surface area of the side faces and top and bottom surface is minimal ifh/b= 1

see picture in the link: https://myalbum.com/album/k1KaEgC1WieE

I must first describe a function / formula with one variable , who can help me thus on the way?

I fail to derive it by myself.