The definition of area usually include the area of a rectangle definition. Can one replace it with "the area of a square of side $a$ is $a^2$"? That is can one find the area of a rectangle in this case?
I've seen something like this in a textbook. Take a square and add $2$ lines to form smaller squares in opposite corners and 2 congruent rectangles. If the length of a side of one of the inner squares is $a$ and one of the other inner square is $b$, the outer square has sides of length $a+b$. So if you take the area of the outer square, subtract the areas of the other squares, and divide by $2$, you should get the area of one of the rectangles with sides of length $a$ and $b$.