Take into account that the side of the missing length is parallel to the side of
12 yd and to the side of 4 yd.
You can notice in the given image, that it is necessary that the sum of the missing length and the length of 4 yd must be equal to 12 yd.
Then, if x is the mising length, you have:
x + 4 = 12
solve the previous equation by subtracting 4 both sides:
x = 12 - 4
x = 8
Hence, the missing length is equal to 8 yd
