An open rectangular box (no top) with a volume 9 cubic meters has a square base. The base has sidexm. The equation of surface area of the box is x^{2 }+ 36/x.

The base costs 1 cents/square m and the sides cost 1.6 cents/square m. I found the expression for the cost as a function of x to be x^{2 }(1) + 57.6/x.

I can't figure out how to calculate the value ofxto achieve minimum cost??? It has to be calculated with the second derivative test.