I'm not certain that I understand your question correctly but I'll give it a try:

1. I assume that you are asked to minimize the material used to form the bottom and the 2 slanted sides of the tank. If so:

Let L denote the complete length (2 slanted sides and the bottom):

2. The cross section of the tank consist of a rectangle and a square which is formed by the two right isosceles triangles:

With a = 300,000 mm˛ you get:

3. Plug in these terms into the equation of L:

4. Differentiate L wrt h and solve the equation L'(h) = 0 for h.

5. Plug in the calculated value of h into the equation of w.

6. For confirmation only: I've got w = 335.56 mm