A three-sided fence is to be built next to a straight section of a river, which forms the fourth side of a rectangular region. The enclosed area is to equal 3200 m2. What is the minimum perimeter that such a fence can have?

I developed two equations: 3600=xy
& 2x + y = P .... Now what????

You need to get one variable:

3600/y = x.

Then insert it into the perimeter equation:

2 (3600/y) + y = P (this way, you only have to solve for 1 variable)

Solve for Y. Substitue the Y value into the first equation of (3600=xy) to find X.

Once you have X and Y, solve for P.