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
-10t - (-17t) - 10t + (-13) = 11 (two negative signs makes a positive)
-10t + 17t - 10t - 13 = 11 Combine like terms
-3t - 13 = 11 Add 13 on both sides
-3t = 24 Divide -3 on both sides
t = -8
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R'S' is equal in length to RS.
length doesnt change, it was just rotated.