Calc question...?

A landscape architect plans to enclose a 3200 square foot rectangular region in a botanical garden. She will use shrubs costing $30 per foot along three sides and fencing costing $11 per foot along the north side.

(a) cost function in terms of x:

(b) minimizing value of x:

(c) the cost of landscaping for this min value of x:

Calc question...?
3200 = x*y

cost = 11*x + 30*(x + 2*y)

y=3200/x

cost = 11x + 30(x+6400/x)

d cost/dx = 11 + 30 - 192000/x^2 = 0

x^2 = 192000/41



x = 68.43 to minimize cost