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 ?