A rectangular area adjacent to a river is to be fenced in, but no fencing is required on the side by the river. The total area to be enclosed is 6000 square feet. Fencing for the side parallel to the river is $5 per linear foot, and fencing for the other two sides is $2 per linear foot. The four corner posts cost $30 apiece. Let be the length of the one the sides perpendicular to the river.

Find a cost equation

Find the minimum cost to build the enclosure: