Question 291111
Let x = one side of the rectangle.
therefore, 2x = other side of the rectangle.
Area is the product of the 2 sides.

{{{x(2x) = 800}}}
{{{2x^2 = 800}}}
{{{x^2 = 400}}}
{{{highlight(x = 20)}}} (disregard the negative answer).

The second side would be 40.

To check:  20 * 40 = 800.  OK.