This is wrong. The cost of the bottom is $3 persquaremeter, not per meter. Theareaof the bottom is square meters. The area of each side, and there are 4 sides, is xh square meters.

I wonder why you thought "3x+ 4h" was the cost.dx/dh = 3x + 6/x^2

then I equate this to zero

which gave me x = 4 and height = 0.09375

The cost of which is $12.375

But this is not a correct solution.

It is a multiple choice question, the selections are:

a. $36.68 b. $45.86 c. $28.68 d. $43.98

Any idea where I get wrong ?