So the problem is:
I have bought a fence 30 meters long and I need to put it around three of my rectangular fields sides. How long should be each of the field sides, to create the biggest possible field size?
My field sides are $a$ and $b$, the perimeter of the three sides is $2a + b = 30$ and the size of the field is $a\times b$. I don't know how to solve this problem, but I think I should use derivatives and get the value of the tangent line to the parabola of the $a$ or $b$ graph. But I'm not sure how to do it.