Let the side of the equilateral triangular base be s, and the length of the
prism be l. Then the volume of the prism is sqrt(3)s.l = 400.
The material used for the package is the surface area of the prism, which is:
A(s,l) = 3.s.l+2.sqrt(3)s,
(3 sides s by l, 2 ends in the form of equilateral triangles of side s)
which is to be minimised.