Question 802102
What do you know about a rectangle?

For yours, x and y are the side lengths, 
{{{2x+2y=10}}}, {{{A=xy}}}, elaborating on this notation, {{{A(x)=xy}}}.



Solve the perimeter equation for y and substitute into A(x).


{{{2x+2y=10}}}
{{{x+y=5}}}
{{{y=5-x}}}
Now substitute:
{{{A(x)=x(5-x)}}}
{{{highlight(A(x)=-x^2+5x)}}}