Question 1009099
w and L, the dimensions, and perimeter is  {{{2w+2L=400}}}.  Simplify this!
{{{w+L=200}}}.


Area is wL, and naming area as a function, you may choose A.  
{{{A=wL}}}.


You want to be more specific for which is the input variable for A.  Look at the simplified perimeter equation.
Either {{{w=200-L}}}  or  {{{L=200-w}}}.  Choose one and work with it.


One way, A as a function of w;
{{{highlight(A(w)=w(200-w))}}};
and using this in this factored form will be convenient.  This A(w) is a parabola with vertex as a maximum point.  The vertex occurs in the exact middle of the roots or zeros, using w as the horizontal axis values.