Answer:
<h2>(6 3/5) * (x)</h2>
Step-by-step explanation:
3x(2 1/5)= (6 3/5) * (x)
The answer is C. 1,000,000
Check the picture below. Recall, is an open-top box, so, the top is not part of the surface area, of the 300 cm². Also, recall, the base is a square, thus, length = width = x.

so.. that'd be the V(x) for such box, now, where is the maximum point at?

now, let's check if it's a maximum point at 10, by doing a first-derivative test on it. Check the second picture below.
so, the volume will then be at