There are two ways to do this. One is by adding up the squares, which takes a while. The other way is if you notice that the length along the bottom is the same as that long the top, and the same is true for the sides. While it does not appear this way at first, imagine that that was the floor plan of a house. If you looked at it from the side, you wouldn't see the dent in the corner, only one side. Since the length of the top is 13 units, from -7 to 6, and the side is also 13 units, from -6 to 7, the answer is 
52 units long.
52 units long.