5. Given that the lengths of the four sides of a rectangle are all integers less than 10, these four length numbers can form a four-digit number, where the thousand's digit and the hundred's digit of this four-digit number are the same, and this four-digit number is a perfect square. Find the area of this rectangle.
(Tip: Let the two adjacent sides of the rectangle be $a$ and $b$, then $\overline{a a b b}=$ $11(100 a+b)$ is a perfect square, which implies 11 divides $(a+b)$. Thus, $a + b = 11$. Upon verification, only $a=7, b=4$. Answer: 28.)