Perimeter is basically when you add up the length and the width. So since the width is 6.5, you can guarantee that there is another side with the width of 6.5 (Because it's a rectangle you're going to have sides with the same measurements otherwise it wouldn't be a rectangle).
So you add 6.5 and 6.5 to get 13
Now we subtract 13 from 34 to get the our length.
34 - 13 is 21.
21 / 2 (because there are two sides that add up to give that amount) is 10.5.
But anyway, the equation would be
(6.5 x 2) + (l x 2) = 34
I hope this helps :)