well first find an equation for the lengths of the sides
so, the total amount of fence is going to be represented by the equation y=2x+6000/x
now fill in the prices to make it a cost function
then you want to find the minimun you can do this by graphing the problem on a calculator and looking at it,or using the derivative to find the min.
fill that value into the cost equation for x and solve an that will be the min cost