i don't have an image so hopefully you will understand.
ok, let's start with the length= 2+4w
we know that there's 4 sides in a rectangle so if you add for sides here's your equation:
(2+4w) + (2+4w) + w + w = 84cm
add all the w's:
subtract both sides by 4:
divide both sides by 10:
l= 2+ 4(8)=34