Perimeter is when you add all the sides together...but oh, no! There are two sides missing! So we have to find it. The horizontal like could be x and the vertical, the y. x would be 3 because one side is 7 and the other side is 4, so if we subtract them, we get 3. Same for the vertical, the y would be 3. (3+3=6)
Now we could add all sides to get the last choice, or 26 cm.