Question 1038661
{{{A(x)=-2x^2+48x=(-2x+48)x}}}


The width is x and the length is -2x+48.


The description and question are not really completely clear.  The 48 meters of fence could be for four sides or could be for three sides using the boundary of the backyard as one of the sides.  Also, you want to maximize A using the variable, x.


{{{dA/dx=-4x+48}}}
Maximized for {{{-4x+48=0}}}
{{{4x=48}}}
{{{x=12}}}---------the width.


Now use the width to find length.
{{{-2x+48}}}
{{{-2(12)+48}}}
{{{48-24}}}
{{{24}}}-----------Length. ... Must be wrong.


Try differently.
Forty Eight meters of fence material, assuming to be used for all four sides.
x and y dimensions.
{{{2x+2y=48}}}
{{{x+y=24}}}
{{{x=24-y}}}
or
{{{y=24-x}}}
{{{highlight_green(A=xy)}}}, A for area.
{{{A=x(24-x)}}}
But this pathway to solve, is not really according to the SPECIFIED MODEL TO USE.  


Going back to the specified model then means the expected answer, again assuming all four sides are fenced using the given length of material,  
{{{x=12}}}
and the area will need to be
{{{-2*(12)^2+48*12}}}
{{{-288+48*12}}}
{{{highlight(576)}}} square meters.