Find the missing dimension: if the perimeter is 6yards and the length is 6ft .what is the width? in yards

Since you haven't identified this figure, I'm going to assume that it's a rectangle.

The Perimeter of a rectangle of length L and width W is P = 2L + 2W.

Here you are given the Perimeter and the length, and are to find the width, W.

Solving the above equation for W, we get P - 2L = 2W.
Dividing by 2 (to isolate that W), we get 

P
-- - L = W
 2

Substitute P= 6 yds and L = 6 feet (or 2 yds), find W (in yards).