Question 811291
Three sides are in lengths x, y, and y.  
Two equations are possible.
Area, {{{A=xy}}}, and fence length, {{{x+y+y=100}}}.  The biggest area A is wanted.


The fence length equation is also {{{x+2y=100}}}, and we can solve for either variable and substitute into the A equation.  Try {{{x=100-2y}}};
Then {{{A=xy=(100-2y)y}}}
{{{highlight(A=100y-2y^2)}}}


That is a parabola with a maximum value.  
Easiest to find the roots of the equation.
{{{100y-2y^2=0}}}
{{{2y(50-y)=0}}}
{{{y=0}}} or {{{y=50}}}, so right in between is {{{y=25}}}.  That is where A is maximum.