Question 176178
Call the width of the rectangle w.  Then the length can be represented by 2w.  Since the area is 648, we have that
{{{w*2w=648}}}
{{{2w^2=648}}}
So, divide by 2:
{{{w^2=324}}}
Now, take the square root of both sides:
{{{w=18}}} or {{{w=-18}}}
(There are always two solutions to a quadratic equation). 
Now, since we are dealing with the width of a rectangle here, negative numbers do not make sense.  So the width is 18 yd.  Then, since the length is 2w, the length is 2(18)=36 yd.