You can
put this solution on YOUR website!To solve this we first need to invoke the formula for the area of a rectangle.
area = length * width
or
area = a*b
We know that one side is

, and the same goes for the other side. Hence we get:

and
Now all we need to do is to multiply a together with b, including the WHOLE expression and setting it in parentheses.
As we may see here:

Continuing, we need to multiply each element in the first parenthesis with all the elements in the second. We then get:
I hope this clarified things for you.
Best regards,
Eric