Question 765079
Let x be one of the sides and y the other.

Perimeter = 2*(x + y) = 27
2*y = 27 - 2*x

Other side y = {{{(27 - 2*x)/2 = 13.5 - x}}}

Area A(x) = {{{x*y = x*(13.5 - x) = 13.5*x - x^2}}}

Hope you got it now :)