Answer :
[tex]x-\ length\\\\y-\ width\\\\
A(x)=xy\\\\
Perimeter=2x+2y\\
24=2x+2y\ \ \ |Subtract\ 2x\\
24-2x=2y\ \ \ |Divide\ by\ 2\\
y=\frac{24-2x}{2}=12-x\\\\
A(x)=x(12-x)=12x-x^2\\\\
Area\ of\ rectngle\ can\ be\ expressed\ as\ A(x)=-x^2+12x[/tex]