xy = 1500000

solve for x in terms of y or y in terms of x (your choice), and substitute into your length equation to get L in terms of a single variable. Take the derivative and minimize.

I recommend you visit this site ...

http://www.math.ucdavis.edu/~kouba/CalcOneDIRECTORY/maxmindirectory/MaxMin.html

... to gain some knowledge of optimization using calculus.