The length is 12 and the width is 6
*work*
a+a+2a+2a=36
(Add up all the common variants)
6a=36
(Simplify by dividing both sides by the GCF*greatest common factor*)
a=6
Now we know that a (the width) is 6 and 2a which is 2x6=12 is the length
Hope this helped a bit