Oh dear, oh dear, oh dear. The rectangular prism is made of steel, so it doesn't require the minimum surface area, it requires the minimum sum of side lengths.
Let S = sum of side lengths.
S = 4(3x + x + h) = 4(4x + 1458/x^2)
S' = 4(4 - 2916/x^3)
For S' = 0, x^3 = 2916/4 = 729, x = 9. You can...