You didn't specify what the question was. However, I am going to assume you are looking for the missing side of the shape.
Let's call the left side "Side A" and the right side "Side B". One is longer than the other side. In this case, Side B is longer than Side A. To find the missing length, you will need to subtract Side A from Side B
Side B - Side A = Missing side
16 - 13 = 3
Therefore, the length of the missing side is 3.
